For the rules of reversi/othello, please see GameHelpReversi
Taking as many pieces as you can from the beginning is the surest way to lose.
The aim of this game is to maintain mobility (that is having as many options available to you as possible). Simply, the way you do that is by having fewer pieces yourself and allowing your opponent to maximise their pieces. This will give you lots of available moves to choose from.
The reason you do this is to force your opponent to make undesirable moves, that is moves that give you a corner for example. Corners are good as they allow you to lock in (or stabilise) your pieces because they cant be taken from you once captured.
Of course if both players are trying to maintain their mobility, it is the person who can do this the best that will win. This is where the strategy in Othello/Reversi starts to unfold.
For another strategy explanation try here: www.youtube.com/othellolessons