How would I go about becoming a helicopter pilot in the army?

Visit the Warrant Officer reqruiting page and start working on a packet.

You've got a long road ahead of you. Once you get your release you'll want mto pick a different job, which is not what you really want, but you'll do that anyway. So, you'll have to wait until the AIT you choose to have an opening. Then you go there for however many months. Then PCS. Now about a year will have passed. Then you can go about meeting pilots to get your LORs. That should take at least another year till your command team is comfortable enough to recommend you to be a pilot.

Or you could do the smart thing, keep your 11B MOS, come straight to a base you choose, work on your letters, and cut about a year off that time line.

AV Branch WO corps is currently very under strength. You may be able to transfer to AD straight to WOCS (Provided you pass the SIFT, 1A Flight Physical, Packer, etc.) Hardest part will probably be getting the release from the NG. Go see a WO Recruiter.