Most of the time you should raise/re-raise with top-pairs (AA-QQ) and top connectors (AK, AQs) in order to make low-pairs and various connectors pay to see flops against you. Remember, they will often have the opportunity to double up on you if they hit (although many beginners do not realize this and fold too often pre-flop).
Stick to the premium hands. You will pay dearly to chase with second-best hands in NL.
Keep most raises down to between 70% and 100% (making it 3 times the big blind to go typically equals an 80% pot bet) in order to save money when you get re-raised or called by stronger holdings. If there are limpers in front of you, raise to about 4-6 times the big blind.