The main tuning slide is the frontmost curved part of your trumpet's tubing. It is easy to find because it is located next to the bell and features the water key. If your trumpet is sharp (you can learn this by using a tuner), inch the slide out a tiny bit until your trumpet is properly tuned. If, on the other hand, your instrument is flat (again, your tuner will reveal this), inch the slide in until your trumpet is properly tuned.