So, let's start by determining the height of our tree and clipping the main stem at the right spot. I like my trees to be about a foot tall. The higher on the stem you clip, the shorter your tree will be, but the more branches you will have available (for making your tree fuller later.) The lower you cut, the taller tree you will have, but the less branches available (unless you devote multiple stems to one tree.)
The picture here shows a side-by-side comparison of an uncut floral stem and one that has been cut. This main stem will be the center of my tree and should have the highest point within the finished tree. So when you're choosing trees at the store, look for ones with tops you like.