Decorating cakes is one of my favorite things to do. I don't do it very often though, pretty much just for my kids birthdays. It's a good thing I keep having kids so I can have more opportunities to make cakes. Most everything I know about cake decorating I learned from my Mother-in-law who is a professional cake decorator and has been doing it for many years. I think the real key to learning to decorate cakes is just doing it. The more you do it, the better you'll get.
The best way to do it is to bake the cake one day, freeze it, and decorate it the next day. So it is about a two day process. I make the cake with a mix and bake it according to directions. Be sure to grease and flour your baking pan well so it will come out all in one piece. After baking let it cool then remove the cake from the pan. I put the cake on a cutting board covered in tinfoil, you could also cover cardboard in tinfoil. Then cover with a large (new and clean) garbage bag and put it in the freezer.
The next day I will make the frosting. Good frosting is essential- I use buttercream frosting. Be sure to get the right consistency so it will hold it's shape. When you pull a spoon from the frosting it should form a little peak that stays up.
Next, I cut the cake out (if I'm going to make it a shape other than the shape of the cake pan). I am not the best artist in the world so I sometimes print out a picture of what I want the cake to be and use that as a template to cut out the cake.
The next step is to apply a crumb frosting. You just thinly coat on frosting to cover the entire cake- it's ok if some crumbs get mixed in and show on this layer. Then you put another layer of frosting over it and make this layer smooth and pretty.
If you need black frosting you can buy it in gel form from a craft store. Gels are nice because they won't dilute your frosting and make it runny (also great for red).
These white connectors are the best! You can also get them at the craft store. By using the connectors you and switch tips and use the same color frosting without needing multiple bags for that one color.
Finally, add any decorations you desire. It's always a good idea to make a border around the base of the cake to hide the frosting that gets on the board around the bottom.
Have fun and be creative!