When you have decided to play your hand and you’re the first one to act or if someone has already limped into the pot, you should raise.
Why should you raise the pot you might ask? When you raise the pot you put in more money and you can also win more money. When you have a good hand you want to get value from your good hand and charge the other players. This will increase your winrate over time. 

How much should you raise preflop? I would recommend to raise 3x the big blind, so if you are playing NL2 and the big blind is 2cent I would recommend to raise it to 6cents. 

I would also raise this 3x on all positions as long as you’re playing fullring. For 6-max it can vary depending on the position you are, but for fullring I would recommend the 3x size.

What if there are limpers?
If there are limpers in the pot I would add at least 1 extra big blind per limper. 

For those of you who don’t know what a limper is, it’s simply a player who limps into the pot by putting one big blind into play. What I recommend then as stated above is to add 1 big blind per limper. Let’s look at it with one example.

Say that you have been dealt AK in a fullring game with 9 players and you’re sitting on the cut off (late position). The action before you is that UTG+1 limps by only paying one big blind.

UTG +2 and UTG+3 folds.

MP1 also limps and all the rest folds until it’s your turn on the cutoff with the AK. In a standard situation if there had been no action before you, then you should raise it to 3x the bid blind. However in this situation we’ve had 2 limpers already, therefore you should add at least 1 big blind per limper. In this example the raise size should therefore be 5x the big blind.
For NL2 it would raising it to 10c ents instead of 6cents. 

The reason for this is that we want to get value from our good hands and increase the pot and the winrate. In general a good poker player raises the pot and never limps. If you’re not doing this yet I would definately recommend it.