In general when you raise before the flop in no limit hold em’ you want to raise to about 2.5-4x the big blind. On later streets I would advise that about 95% of your bets should be sized at anywhere in between half and the whole pot.
Before the flop you want to raise the same consistent amount with all of your hands so your opponents will have a hard time reading what you have. If you usually raise 4x the big blind, but when you have AA or KK you suddenly only raise 2x to keep people in the pot, or 6x so you don’t get outdrawn you are giving away way too much information that players will easily pick up on. I like to raise 3x the big blind if no limpers have entered the pot, and I may add one big blind to that raise for each limper to compensate for the fact that they have made the pot larger. Beginning players must remember that being consistent before the flop is the important part, as it will keep your opponents from getting a read on you.
On the flop you should bet between half and the full pot almost 100% of the time. On later streets there may be strategic reasons to overbet, or underbet, but in a standard limped, or singled raised pot it is best to keep the bets in that range. I see many players overbet the pot with a hand like top pair, and I’d like to explain why this is wrong. I was playing live when the player to my right bet $30 into a $6 on a flop of 832 with the hand 87. He justified it by saying that he was protecting his hand, while in actuality what he was doing was turning his top pair into a bluff. Think about it, would any hand worse than 87 ever call such a big bet into such a small pot? Of course not! So if only much better hands were going to call, many better hands would fold, and there was no possibility of a worse hand calling, this must be a bluff. The player to my right wasted a lot of money with his large bet if someone in the pot had a very strong hand like 33, when he could have accomplished his same goal of protecting his hand with a $6 bet.
On the other end of the bet sizing mistake spectrum is underbetting the pot. A beginning player holding AK might bet very small on a flop like KJ8 thinking that they are suckering their opponent into the pot, while in reality they are the sucker by allowing opponents to draw cheaply, and by not getting maximum value out of their strong hand. With a hand that is likely to be the best this beginning player should not be afraid to put in a pot sized bet to protect his hand from possible straight draws, or a player with a lower pair catching up by making two pair or trips. An experienced player with QJ or AQ could easily call this small bet looking to get paid off big time if he catches up, while only losing a small amount of money if he misses.
