Push yourself, try and see how many push ups and sit ups you can do before your body just gives up. Try and do this every time you workout and see if your endurance has gone up over time. You should be impressed with what your actual limits are after a few weeks of constant exercise.
A good nutritional tip is to keep eating healthy even when you're trying to put on muscle. A lot of people think they can eat whatever they want when they try to gain muscle, but that kind of reckless eating will only result in fat gain. Instead, keep eating healthy as you normally would.