I'm 5'7" and 165 lbs so I'm at this point where I should do some lifting as well as some cardio to lose my gut. I'd like to get down to 150, ideally, maybe 145 or 140.

Anyway, after my workouts would it be all right if I took some whey or should I just avoid the calories? I usually have a can of tuna fish for a sandwich afterwards so I'm missing out on any protein.

A good whey shake has very little cals.

You should figure out your nutrient ratio in order to determine if you're meeting your needs. If you don't have enough protein, supplement. When used correctly, protein supplementation is a powerful tool. When used in ignorance, it will be anything from a waste of money to a substance that's detrimental to your goals (over consumption will help you put on fat).