If there is one raise in front of you, regardless of position, play only TT+ and AK. If there is more than one raise in front of you, play only KK+ (i.e. 3-bet or 4-bet with KK+).
Usually we want to bring it in for 3-5BB. If there are only limpers in front of you, raise to 3-5BB plus 1BB per limper (if you have a hand that you would normally open with in that position). In general, we like to raise more in early positions and less from later postions (if this confuses you, simply always raise to 4BB+1BB/limper).
If someone has raised in front of you and you have a playable hand (TT+, AK), usually go all in. If your stack is larger than 30BB-35BB (i.e. becoming a medium stack) or when there is only one raiser and the raise was the minimum, make the largest raise you think your opponent will call.
When you get reraised by someone behind you, usually you should go all in or fold. If you have QQ+ or AK, go all in. Go all in with any hand if your (or your opponent's) remaining stack is less than twice the size of your original bet.
There are a few more hands we can play when there are just limpers. If you are on the button or cutoff, after only limpers, call with 22-66, A2-A9 and any two suted cards that are both ten or higher ( i.e. KJs, KTs, QJs, QTs, JTs). If you have a stronger hand, be sure to raise as described above.