Looking to switch from AT&T - Some questions

Hi All -

I'm currently on the home stretch on my contract with AT&T. I plan on paying the early termination fee and switching to the $30 prepaid plan (100min/5gb data). I'd like to keep my number and bring it over. I'm on an iPhone 5 that is still locked to AT&T.

What would be the first step I should take?

Would T-Mobile's early termination fee payoff applicable to my situation?

Can I finance my phone through T-Mobile even with a prepaid account?

Any other insights or advice before I do this?

Thanks!