• I’m totally on board. • I’m in favor of it. • I’m all in. • Count me in. • I fully support it. • I’m down for that. • Sounds good to me. • I’m game. • I back that. • I’m with you on this.

Depending on context, some of these may be suitable or unsuitable alternative phrases: Sounds good to me. I'm up for that. I'm completely onboard with it. That's a bit of me, that is. I totally support it. Count me in. I'd love to see that happen.

I totally agree.

The answer will depend on the context, but there are numerous alternatives: I agree completely. Right on. Yes, 100%! ...and so on. Again, I would like to see more context. I hope that helps!