The shoulder makes the arm I always say, so in order to get that nice round shape, you have to hit all three head of the shoulder. Shoot for 15 reps and pick a weight where you begin to struggle around 12 reps. If you need to rest a moment to finish them, that's ok.
You want the body, the fitness, the health but you're just not sure you can.
How do you believe in yourself when you don't believe in yourself?
You have to DO what needs to be done, regardless of how you think you'll do.
You can be eating stark white clean and working out until your limbs are numb, but if you don't plan how to actually reach your goals, you're just taking a chance you'll ever really get there.