First and foremost I learned how to make icing:
beat together-
1/2 cup softened butter
1/2 cup crisco
1 tsp. vanilla
gradually add-
4 cups powdered sugar
then add 1 tbsp milk more or less if needed.
To be honest I personally don't like the flavor of this, it just works really well for decorating.
Who knew there is even a special technique for frosting a cake (Basically avoiding crumbs to show on the surface)? I would then, if you don't already have them, get some disposable piping bags at least 12", they come in like packs of 50 or so. These come in handy when you want to lets say... make red roses, green leaves, and purple writing all in one cake, then you don't have to keep cleaning one bag. After I make frosting I decide how many color I want, then separate various amounts of icing into smaller bowls. When I add color I use a toothpick to transfer the dye, then I'll mix it to see the resulting color, if it is not strong enough I use another toothpick and add more, never reuse the same one if it has touched the icing already.
I first started practicing with the star tip doing drop stars, and different borders, just play around with applying different amounts of pressure. Then I did some very simple lattice work, and practiced writing which for me is difficult. These two tips I use the most so I would start just practicing with those.