Push ups - Start off doing standard pushups. Hands should be at shoulder with and your back and rump should be straight and not arched. Get to where you can do a decent rep of these (20 +) comftorably. Then try knuckle pushups, slightly harder because of the extra elevation and slight turn of the forearms to compensate for going to knuckle positon. When you get good at these move to diomond pushups (standard push up position except hands should be together and index fingers and thumbs should meet and form a diamond where your nose would touch the floor) and jumping push ups (standard push up except when you push up, you do so with enough force to lift yourself off the ground, quickly clap and then catch yourself just before coming to the ground in the standard position).
Sit ups - do a normal rep and then later do another rep except cross over. Make your right elbow touch your left knee, go back down and then come back up except left elbow to right knee. That is one rep of cross over sit ups.
Squats
Crunches
Deep knee bends
Cant remember the name of it but you can squat at your bed or a bath tub with your arms at 90 deg behind you suporting your body on the propose objects. Now push yourself up and resist coming down only using your arms.
Also work on cardio. Just going out and taking a lap or two around campus at a jog can help out.
You may also want to invest in a set of dumbbells. But before doing that you need to ask yourself what your focus is. Do you want to build muscle or make lean muscle? Building muscle you take heavier weight at low repetition and lean muscle takes lighter weight but at a much higher repetition.