trim the motor in when you are starting off and want to get up on the plane. Once up on the plane, trim it out. Your ideal trim setting is determined a bit by the conditions. On a flat day you can trim it out until it starts to cavitate. You can determine the optimal setting by the sound of the motor and also you will find as you trim it out that the speed will increase to a point. In rougher conditions, you will not want to trim it out as far to prevent too much bouncing of the nose.
As you get more time up on the boat, you will get a better feel for the correct trim setting.
Jeremy