Communicate with the tower continually. The tower must know that you're doing a go-around, so that the controllers can organize the other planes on the runway. Since you'll probably be flying by sight, you'll need the tower to help guide you through traffic.
Go to full power. During your descent, you should have slowly cut the power with the intention of coming to a stop once you hit the runway. To complete a go-around, immediately apply full power to gain momentum and start pulling up on the yoke. You should begin to climb like you would during takeoff.
Watch your flaps. Once your flaps are fully extended, retract them in 10-degree increments until you have a positive rate of climb. Cessnas have excellent flap control; you can't retract your flaps more than 30 degrees in newer aircraft or more than 40 degrees in older models.
Fly slightly off the runway center line in order to see any other aircraft. Circle the airport in this manner, relying on sight to check for hazards and oncoming aircraft. Once the tower has cleared you, you can go in for a landing as you normally would.
