These often grow as multi-trunk trees. Next year at the beginning of winter, you can remove any branches that you don't want. You'll want to select the three healthiest shoots and keep those. If you'd like a single trunk tree, select the healthiest shoot and keep that one, continue removing any new shoots as they appear. 

I would strongly recommend protecting the tree with chicken wire or fencing, as rabbits and deer enjoy chewing on the tender shoots.

https://hort.extension.wisc.edu/articles/river-birch-betula-nigra/