Supplements will NOT build muscle without proper stimulation, from exercise. They will help fill in any deficiencies in your diet so you get maximum benefit from your exercise routine.
First you need to understand your macronutrient breakdown (protien, cabs, fat). Most people shoot for 40/30/30. If you have a deficiency you can correct that with supplements. Most people tend to have trouble getting enough protien, which is why protien supplements are popular. Fish oils help with "good" fats.
You will never build much muscle without an intense resistance exercise regime year round. I would recommend you try to do some sort of reistance exercise year round, even if it is only a few sets of a few exercises. A pretty complete exercise session only requires 3 exercises from 6 major movements:
1. quad dominant (squat, stepup, etc.)
2. rear chain dominant (deadlft variations, good mornings, etc.)
3. horizontal push (bench press, pushup)
4. vertical push (military press, handstand pushup)
5. horizontal pull (row)
6. vertical pull (pull up)
If you do 3 of these in one session and 3 in the next session and alternate them you can get a complete workout in a very short time (20 minutes each session), 2 sets of 12-15 or 3 sets of 8-10 reps is all that is required to maintain and make progress.