If you want to concentrate on your triceps with pushups, make sure your elbows are tucked well into your sides and your hands are parallel with your body. The more you turn your hands inwards, the greater will be the involvement of your chest muscles. Ideally, you should try to spread the strain over both muscle groups, but you can strongly influence this with the position of your hands. It stands to reason that the more pushups you do, the stronger your muscles will become.
Forearms: Take one piece of round wood about an inch in diameter (a broomstale is ideal) and some 18 inches long. Drill a hole through the diameter at the halfway mark and thread a four-foot piece of paracord through it. Tie a weight (about ten kilos should do it) on the dangly end and simply roll it up onto the wooden bar with both hands outstretched in front of you and gripping the wooden bar until it's at the top. Then let it down and roll it up in the other direction. What you're trying to do is use the inner muscles of your forearms for one direction and the outer muscles for the other.
Of course, if you're only three-foot-six, you'll have to shorten the paracord.