Buy a tree and set it up. Choose a color scheme, such as all white, blue and gold, or red and pink. You may want to take a look at the ornaments you have before you choose, to see if one or two colors predominate. Put lights on the tree. Collect the materials you'll need.
Ornaments come in many colors, and many stores sell single-color boxes of icicles and glass balls.
Remember that anything is fair game. If you're a football fan, choose your team colors - and put the helmet at the top. Buy some ribbon to tie everything on. Any string or cord will work, but ribbon comes in colors to match any color scheme. Attach the decorations to the tree, starting with the biggest items.