There are three looks you can go for with the crape myrtle: The Single Trunk, The Multi-Trunk, and the Natural Look.
To get the beautiful single trunk, you will need to remove any extra stems that are producing from the ground, called suckers. Also, you will need to trim any new growth that appears on the main trunk that may turn into a branch. Focus all the branching to the top quarter of the tree.
If you want the multi-trunk, allow your crape myrtle to branch along the length of the stem. Each growing season allow a few suckers to grow into stems.
The Natural Look is the easiest because there is little to no pruning involved. Allow your crape myrtle to branch and spread naturally.