Fighter jets are designed for speed and maneuverability, not comfort. That means they don't have many of the amenities you might find on a commercial airliner, including toilets. Instead, pilots relieve themselves using a relief tube or piddle pack.
The relief tube is a funnel-like device that attaches to the pilot's flight suit. The pilot urinates into the funnel, and the urine is collected in a bag. The piddle pack is similar to the relief tube, but it also has a small seat for the pilot to sit on while relieving himself.
Some newer fighter jets do have toilets, but they're usually reserved for larger aircraft like bombers and cargo planes. So if you're ever lucky enough to ride in a fighter jet, don't expect to find a toilet onboard!