That means sharpen the knives if possible or replace them. That's intimidating for some, but there are videos on You Tube to show you how.
Set the fence at 90 degrees to the table.
Make sure the guard is working properly and will return on it's own.
Run a straight edge from the outfeed table to the front or infeed table and see what the depth of cut is. It should be 1/16" . Raise the infeed table if it's more than that.
Have a push block with a "hook" handy, like the one Steve posted above. Push pads with "grippy" bottoms should be cleaned with lacquer thinner first to remove any residue, so they will infact grip.
If all that is OK, then proceed as follows:
Lay your work on a flat surface to see if it rocks. If it has a slight twist it will rock slightly, so be prepared to hold it at a constant angle when you first start out.
Think of the jointer as a powered hand "plane". You want to remove material where it is not needed by planing that portion away, thereby flattening the surface. It's OK to partially plane from one end , turn the piece around and plane in from the other end.
Make marks with chalk if your progress is difficult to read in the wood.
Face joint your work first to get a flat surface on one side. Don't start with a board that is less than 18" long. Run the board into the cutter slowly and listen to what is happening. It may sound like it's cutting off in some places and not in others. That's OK, you are flattening the surface.
Once that surface is flat, you can edge joint one edge by holding the flat surface against the fence and feed the edge into the cutters. Keep a constant pressure against the fence and feed slowly.
The whole point of jointing (straightening and flattening) a board is to prepare it for either the table saw or the thickness planer which is the next operation.
You should NEVER run a crooked or twisted board on the tablesaw since it can twist or shift, jam and cause a kickback.