You can gain streght with doing only push ups. Easy way to increase resistance is to put a box under your your feet, the bigger the box the bigger percent of your bodyweight is on your arms. When you feel strong and comfortable doing push up with the box under your feet, you can take the next step and put smaller boxes under your hands. That makes the movement longer and much harder to do.
This was just an exercise you can do instead of bench press, with my english it would take a week to tell you about training the whole body. But you can become stronger without going to gym, by doing one legged squats, pull ups, push ups, sit ups etc. With little changes you can make the movements either hard or easy, depending how many reps you want to do.
And don´t worry about the numbers, do as many reps as you can do with good technique, and don´t do sets to the absolute end.
Dixi et animam levavi