When to put them in?

[Erikmyg]

Hello 2+2
I am quite new to NL (just recently converted from FL) and I quite often find myself in dilemmas regarding when to put my chips in. Too often i have called all-in raises with TPTK and lost to stuff like unlikely two-pair combinations or runnerflushes.
So my question is, if there are som "official" poker guidelines regarding this topic, like "never go all-in with less than two pair" or whatever. Or is it simply situation-dependent to say anything concrete?
If it helps, i play 50NL at Party Poker.