You can volunteer by asking your HR for a list of units that have been sourced or around to receive their mobilization orders. You would have to do a release and acceptance, they just can't take anyone.
You can't volunteer for Active Duty (unless it's Mobilization. ADOS/AGR is still Title 32 which Active Duty is Title 10). You would have to fill out a DD/DA 368 and pray your Chain of Command signs it.
There is a link through GKO (Guard Knowledge Online) where you can volunteer for deployment. This is not strictly for deployment to a war zone, you could be activated for the border guard duty or any stateside mobilization also.