Supplies
I will not tell you to run out and buy every piece of equipment and every cake decorating tool available at this minute. But I will tell you that certain tools will make the job easier.


Icing bags
Featherweight - These are a lightweight polyester bag that's reusable and come in a variety of sizes. These bags are more expensive, but they are easy to use and last for a long time if cared for properly.
Disposable - You can purchase plastic, see-thru bags that come in a variety of sizes from a multitude of manufacturers. They can usually be re-used a few times before they need to be thrown away. They are less expensive but have a very limited life-span.
Parchment - parchment can be purchased pre-cut and boxed, or you can buy parchment paper and cut your own triangles. It's a little bit of a hassle to fold your own bags, but they're the least expensive and disposable. I also find it's more difficult to re-fill parchment bags. I do use them for colorflow.
Vinyl - Every once in awhile I'll see decorating bags on the market that look very similar to the featherweight bags. These bags are much heavier and made of a tougher fabric. The problem with these bags, I've found, is that they're more difficult to squeeze and it's much harder to twist and close the back of the bag when piping.
Other. In a pinch, I've used wax paper and ziplock or sandwich baggies for piping icing. It's quick and easy, cheap and disposable.
Icing tips - As you decorate you'll find that certain tips you need more of than others, depending on the types of cakes you're doing. If you're doing a lot of cakes with star fill-in techniques, then you'll need several #16 tips. If you're doing a lot of cakes and don't want to be doing a lot of dishes in between, sometimes it's just easier to have multiples of each tip. Where do you keep your tips? Do you have a tip case? A box? Are they all mixed together? I like to keep my supplies organized. I have a box like this, with labels on the front of the drawers where I can easily access and find my tips all the time.
Turntables - There are multiple sizes and styles of turntables on the market. Some sit close to the table, while others are higher on a pedastal. There are electric turntables, rectangular turntables, tilting turntables and some made especially for fondant. While not absolutely necessary for decorating a cake, they are extremely helpful, particularly when actually icing the cakes.

Spatulas
Straight - straight spatulas are straight with plastic or wooden handles that come in a variety of lengths and widths. While used in icing the whole cake, they are particularly helpful when icing the sides.
Angled or offset - The first four spatulas to the right are offset spatulas. These are used most when icing the tops of cakes. They allow you to ice the cake with the flat of the spatula held flat, without having to drag your knuckles through the icing.
Tapered offset - The very first spatula in the picture is a tapered spatula. The end of this spatula comes to a slight point which is helpful when icing into corners, or having to remove sections of icing.
Couplers - Couplers are a 2-piece tool that fits into an icing bag and allows you to attach and change tips on a bag of icing. While it is possible to place your tip into the bag and then add your icing, you will be unable to change this tip should you need to use that icing color for another technique that uses a different tip. I highly recommend having multiple couplers handy.
Fondant smoothers - I use fondant smoothers for smoothing both buttercream and fondant. They're a smooth, flat, plastic tool with a grip that allows you to smooth your icing and fondant by putting pressure on the cake to "iron" out the wrinkles. While this is not absolutely necessary, I find that this tool has been quite beneficial to me.
Icing colors - There are several brands of color on the market. Some are liquid, some are paste or gel, and some are powdered. You can even buy color in a can and airbrush colors, as well as pearlizing and metalic colors.
Generally speaking, when coloring icings, you'll use a paste or powder food color. If you add liquid colors to your icings, it will thin it down and change your icing consistency. Most gels will not unless adding a great deal of color to achieve a very dark icing. If this is the case, you may need to add additional powdered sugar to stiffen your icing back to it's usable consistency.
When coloring your icings, use a toothpick or a clean spatula and add a little bit of color at a time until the correct shade is reached. Pastels will require very little color, while reds, blacks, browns and other dark colors can require quite a bit. Too much food coloring can make your icing bitter, so here are a few tips for making darker colors:
Use a "no taste" food color if available.
When making black icing, start with chocolate icing first. You'll need much less color to make it black.
When making red, use a koolaid packet of color to get it started.
Don't try to make your icing the full depth of color right away. For instance, for black - color your icing a dark grey, and usually after sitting for a while, it will darken. This is the same with red, navy and any very dark color.
Powder colors don't seem to have as bitter a taste.

Cake boxes
Many decorators can put out a pretty cake, but if it's not displayed well, it will lose value, particularly in the eyes of a paying customer. Cake boards or cake circles can be purchased from most craft stores, as well as many party supply stores and specialty cake supply shops. Shipping can be cost prohibitive on cake boards and boxes due to the weight, so if you can find a local vendor, even if paying a little more for the items, you'll probably come out ahead in the long run.

There are a variety of boards on the market; plain, waxed, gold foil, scalloped, etc. How do you know which to use? Sometimes it comes down to cost and sometimes just preference or ease of use. In my opinion, a cake does not present well on just a plain board, or even one covered with aluminum foil. They tend to look tacky, cheap and non-professional. (Even if you ARE non-professional, you should still want to put out a product that looks professional.)

Waxed boards will keep the moisture from the cake and icing from soaking into the board, but it's still a plain board and does nothing to enhance the look of your cake. There are some waxed boards that have a design on them that aren't quite as plain, but they come in limited sizes and styles.

Gold foil boards are more costly, but they take less time to prepare. They come in a variety of sizes and shapes. They also have an unfinished edge which can be unattractive.This is where a decorative foil can be handy. You can purchase decorative foils for cake boards in a variety of colors. There are some that are specifically made for this purpose and come in white, gold and silver and are FDA approved to be used with food products. A roll of this foil can be pretty expensive, but when comparing the cost of plain boards covered with foil to the cost of purchasing boards already "finished", it is relatively comparable.
Many cake decorating supply stores will also carry a variety of foils known as florist foil. This foil is NOT FDA approved, but is also considered non-toxic. Many decorators will use this foil, but will take care to not have the cake sitting directly on top of the foil. This foil is moisture resistant and comes in a variety of colors, and is less expensive than the others on the market.
I have also seen decorators who have covered their cake boards in wrapping paper. Please keep in mind that wrapping paper is also not FDA approved, nor is it moisture resistant and it will discolor fairly soon after putting your cake on it. However, it comes in an endless supply of colors and designs for any occasion.

How to cover a board with foil
So - now you have a board fit to display your cake on. Now what? I think my biggest decorating pet peeve is the person who then sticks toothpicks into their cake and covers it with plastic wrap! Let me clarify, if this is for your own family and you don't want to go to the extra expense of a box, then invest in a cake saver to keep and transfer your personal cakes in. It looks nicer and will keep your cake from drying out as quickly. Or, you can even use the same cardboard box over and over. If you are selling this cake, it needs to be boxed properly. You want that cake to get where it's going safely and you should want to be professional in the manner in which it is viewed.Please remember that when you're figuring your cost for your cakes, always work the cost of the boards, foil and boxes into your cost.