Walk through those rustling leaves.

When you start to play the game you will often notice rustling leaves, for the most part people just pay no attention to them, but in fact the rustling leaves are where there are pokemon hiding, so walk through one of the patches of rustling leaves and pokemon is bound to appear.

Different areas house different pokemon so vary your location.  But remember when you see the rustling leaves make sure your characters circle touches them to gain a pokemon.