This shit happened today at my gym.
If you do curls at the squat rack, I don't care how many sets, you are a fucking asshole. There are 400 ways to do curls, don't take the squat rack to do them.
@OP Since you're a thin dude, get a gym membership and start lifting. Squats, Deadlifts, Bench, Pullups, and then whatever else you feel like doing. Use free weights, do not use the lame-ass Smith machine. Cardio as you see fit. Use light enough weight to keep good form, but if you can do more than 10 reps in your first set you're lifting too light.
If you want to up your bench, this is by far the best routine to follow: Muscle Media's Benchpress Routine - tells you exactly what to lift given your current 1-rep max. It's fucking awesome.
I do squats in 5X5 (5 sets of 5 reps). Try to find a starting weight you can do 2 or 3 sets of 5, but not 5X5 (with good form, bending your knees so your thighs are parallel to the floor) and then keep doing that weight until you can do 5X5. Then up it 10 lbs next workout and keep going.
Good luck!
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