Most tree-form hydrangeas are trained that way by professional growers but it is possible to do it yourself. It works best if you can start with a very young plant -- often done with cuttings that are restricted to single stem. From this point, you just allow the plant to grow vertically (helps if staked), removing all side shoots until you achieve the desired height.

What you need is a clear expanse of trunk or a single stem which can be trained upright and allowed to develop into a sufficiently woody structure to support the topgrowth. I'd remove any small lower branches now and also stake the selected upright stem to begin its training vertically. I'd probably remove additional side shoots come spring also. Make sure you leave enough new growth at the top of the plant to provide adequate photosynthesis.

This is a long, drawn-out process, so have patience :-) That's why already trained tree-form hydrangeas carry such a hefty price tag compared to the shrubs -- time and hands-on attention!

Here is a link that might be useful: training shrubs into tree forms