You need to make sure it is:
- 100% Whey Protein
- Has added Leucine
- Added Glutamate (optional)
- Has a low carb/fat percentage (depending on your goals), if you are trying to bulk up them medium carb and low fat is OK
- Like the previous posters have said, timing is most important, no longer than 30 mins after your workout. Best to consume with milk or have some good slow release carbs for refueling glycogen stores
I do know GNC has a good range of 100% Whey with added Leucine
Also would recommend taking Creatine Powder separately for energy purposes. See tub for dosage requirements.
If you are just starting off I wouldn't say you need to spend more money buying a Casein Powder, just have a cup of cottage cheese before bed each night would suffice.