This is a good video showing some of the basics on cutting in the walls and windows while painting your rooms. Taping is very tedious and time consuming so I would recommend learning to cut in this way in a closet or room that people won't see where you were practicing.
1 tip I will add is, put the brush an inch or so from the edge. when you press the brush against the wall, the bristles will spread. When the bristles are spread to the edge, then start the straight line.