Basics of buying a new phone?

So I'm buying a new phone in stores pretty soon, and I've never done it. I also do not plan on doing the monthly pay. So my question is

1.) Do I have to do the monthly pay?

2.) Can I just pay all at once?

3.) Whats the process like when buying a phone?

4.) How long would it take to finish the process?

*I am already a t-mobile user as well

Thank You