simple(for some), but I need some help

[smartalecc5]

I got a question that comes up every few hands or so and I still don't know what the heck to do. If you could explain yourself that would be great - rather than just replying call or fold.

Ill just give two examples, so you know what I'm talking about.

#1: Hero is on Button with 57s. UTG raises, MP 1 calls, MP+2 folds, Co CALLs, HERO???

#2: Hero is SB with 24o. UTG folds, MP 1 call, MP2 calls, Co calls.

And countless other vcariations. How do I decide what to do?? Thanks for helping me out with my noobish question.