In order to get a taller tree, it must be pretty much trained that way from the start. You need to pick/have a central leader, much like a tree in your yard, and encourage growth in the upright direction. You will most likely need to stake and tie it, if you want symmetrical upright growth. You can pinch the growth out on the lower branches to divert energy to the main leader, but you'll need to allow some lower growth to develop a solid trunk, and prevent leggy growth. Think of it like a miniature tree in a yard, where the main leader goes up, and as it gets taller, you trim the bottom out. Typically you prune out the bottom 1/3 and keep the top 2/3 full. Just remember, it's a loooong process to create a nice tree from a small plant, so unless you intend to work on this for many many years, you may want to buy one that suits your needs.

Just a quick side note: search your local plant shops for deals, particularly "dead" or "dying" plants. I scooped up a nice ficus alii straight from a plant shop dump cart, on its way to the dumpster outside, for free. It was 4 feet tall, and about a 1.5" trunk, just what I wanted. I rehabbed it, and after about 3 years of work, it looks like a plant you'd find for $50 or more.

Joe