Let me give you something to aim for. A a co-worker of mine (jamacian woman, around 25 y/o) joined the army, got sent to Afganistan, and by the end of 12 months, she could do 50 pushups. (I can barely squeek out 15).
But stay at it, Blink! You’re already ahead of the game because most folks dont excersize at all.
Another good idea is crunches. Lie on your back, hands behind your head, and raise your knees to your elbows; then straighten your knees till they are paralell with your back, but and dont touch your heels to the floor till you to this 5 times (aim for 10 as a goal). Much easier on your back than situps, and accomplishs almost the same thing.