Stretched canvases usually come pre-gessoed, but the thin gesso layer applied at the factory is usually insufficient to provide the best foundation for subsequent layers of acrylic paint. To prep your canvas properly, plan on applying at least two to three more layers of gesso. (My preference is for three, at minimum.)
Apply your first coat of gesso to the canvas with all brushstrokes laid down horizontally. Let dry thoroughly. (If the surface feels cool to the touch it needs more drying time.)
Once it's completely dry you may sand it very lightly with fine grit sandpaper or a piece of brown paper bag if you wish to paint on a very smooth surface. This step is entirely optional. Before going on to the next step be sure to remove any dust by wiping the surface with a cloth dampened with water then wrung almost completely dry.
Next, apply a coat of gesso in a vertical direction. Allow to dry thoroughly, then sand lightly as above, if desired. Again, remove all traces of dust before proceeding.
The third coat of gesso should be applied in a criss cross fashion. Once dry, you may sand it lightly or not, the choice is yours. Now your canvas is properly prepared, and you are ready to paint.