This rule is easy and logical, but it needs to be mentioned. This chapter will be a breeze and soon you will be able to play!
It’s simple. You must not (nor would you really want to) kill your own stones.
A stone that immediately has no liberties (or fills the last liberty of your group) cannot be placed unless the move captures at least one of the opponent's stones next to it. In this case it is allowed because the capture gives you at least one liberty.
Black to play
You are not allowed to place a stone at A or B, the placed stone would immediately have no liberties. You can, however, place a stone at C because you capture the neighboring white stones with that move, thus giving your stone liberties. Try it.