Well, you can do 1 of 2 things:
1. You can beat your opponent by the score. This is pretty basic, just hit uke/a player as much and as hard as you can until you have more points then them. Kicking is a popular way to do this, as well as aiming for your opponents head.
2. You can disqualify(dq) them. This happens when any part of the body touches outside of the dojo(if there is one), or if anything besides your hands or feet touch the ground, if dq is on(you'll learn about that when making a server). This is done by grabbing uke(clicking the hands, NOT the wrists), then forcing uke to the ground, and making them dq. This way is harder then #1 for newer players, I believe.