What to do with a japanese zelcova tree?

I bought this mini tree at Ikea because instructions said it was easy to care for and it looks awesome (w/o the price of a bonzai tree). It started sprouting a ton of little green branches. Am I supposed to prune them? Is this going to stay little, or...

should I plant it in the yard soon? I am so surprised I haven't killed it yet after a whole month! What do I do with this thing?

Ooh, you are now getting advice from someone who gardens by trial, error and hard-won experience, not through knowledge and wisdom... However, yes, my 'trial and error' would be to prune back any of the branches that stop it being the shape you want. I don't think it will harm the plant.

Bonsai stay small because their roots are trimmed as well as their branches - it's a real hassle. If you want yours to stay tiny without the root trimming stuff I think that you are best keeping it in a small pot, which should restrict its growth. If it was sold as a house plant then you should be OK with it indoors. Has it grown a lot?