The best advice is to practice and also watch some training videos or read articles to understand, how other good players handle common postflop situations. There are many good hand examples in the CC 30 day course:
You can also share
hands, you feel, you struggle with, to the tournament or cash game hand history forum and get feedback from other more experienced players on those specific hands. If I am to make a few select points it would be:
* If you are the preflop raiser, you should be more inclined to C-bet wide, if the flop is good for your range like



or



rather than



or



. You can still C-bet the latter boards, if you have something, but you should not C-bet, just because you can.
* In multiway pots its generally best to give up with marginal hands. If you have


on



against 3 opponents, just check and fold. Chances are, you are behind to at least someone, and even when you improve, sometimes you just set yourself up to lose a big pot. If turn is

, now you have TPTK, but someone else might still have you beat with hands like


or


or


or


.
* If you are really not sure, its better to make a small folding mistake than a big calling or betting mistake.
* Learn to think more in equities rather than "missed/hit".


is a little better on



than


, because at least you have a BDFD.