Jointing is done to smooth out and straighten a single surface of a board. Once you have a single straight surface you can straighten out the other sides to create a near perfect board.
Make sure before you run your wood through the jointer to find the direction of the grain and cut along that direction to prevent tearing up your wood.
When pushing your wood through the jointer provide lateral force to keep the wood moving along the jointer blades evenly.
The jointer will only ever take a small portion of the surface off so you'll have to run your piece through several times to provide a clean smooth surface
Once done jointing the first surface of your board to the point where its smooth, joint a edge using the smoothed as a guide surface to provide you with a perfect 90° edge.