This is one tiny problem i had when i started playing the game. Its a very simple thing but… Anyways, you can use the Space bar to jump. but of course you should first go near some obstacle (that is low enough for you to be able to jump over it) and then take cover (by pressing space bar) then you should press the space bar again. take note to do this quickly. when you press the space bar for the second time your character should perform a jump.