Pressups, inclined pressups i.e. put your legs up on a wall or something and then go down (It's alot harder .
Press ups where either your hands are next to each other centering your body on the floor or widely spread apart.
Use your imagination and once you feel your arms ache and maybe hurt a little.. you're doing the right thing.
Use the internet foool there's plenty of exercises out there.
hi you could also try doing things like bench dips (you can use anything sturdy instead of a bench though ie end of bed, wall) these will strengthen your triceps so theyll help you with press ups ...
(you can try spider man press ups *text deleted*classic press up but as you lower your body bend your right leg and rotate your knee out until its outside your right elbow then repeat on other side*text deleted** )
also as said pull ups, you could do chin ups then for your core youve got sit ups, crunches, leg raises, plank, single leg lowering, side bridge..