It's a little hard to figure out where to begin with this one. There are so many different ways to decorate a cake! I've chosen things that I feel are both simple, especially if you're just beginning to learn this craft, but also effective so that even if these are the only techniques you learn, you will still be able to create beautiful cakes.
Cake Design
I would recommend that you start by looking at images online. Use Google Images to search for cakes using key words, for example "girl 1st birthday cake", "retirement cake" etc. Also, most cake designers have websites with galleries full of images. These are also wonderful sources of ideas.

As you look at the images, deconstruct the decorations and figure out if you have the skills to be able to create a specific type of design. There's nothing worse than wanting to create a cake that looks a certain way and then not be able to do it because you don't have the ability! I learned quickly that my piping skills are horribly lacking and because I make cakes on a by-need basis I also don't practice, which means I avoid designs that require piping.
Once you've looked through images, decide which design elements you want to incorporate and what look you want to give your cake. Then do a quick sketch on paper to see if you are able to get the design you're thinking of out of your head. It doesn't matter if you're not an artist, it's just helpful to see your concept on paper because you'll be able to evaluate it from a different perspective.
Once you've finalized your design, gather the tools you'll need and begin!