how do i play my chosen champ and does it go straight to bench or can i also put it in battlefield

You play your Chosen Champion exactly like any other unit: pay its cost from the Champion Zone, and choose the location it enters — your Base or any battlefield you already control. It's not locked to Base.

It enters exhausted (unless you pay Accelerate), and it can't go to a battlefield you're attacking/contesting, since a contested battlefield isn't one you control yet — units played during that combat go to Base instead (Rule 355.2.a, FAQ #3843). The Champion Zone is basically a revealed extension of your hand: no abilities usable from there, and you need the Reaction keyword to play it during a Closed State.