If it has a 'Clean' setting, run it for an hour (or three). Usually there is a lever that has to be engaged to ensure that the oven is tightly sealed.
If there is no 'Clean' setting, then run it for an hour (or three) on the highest temperature available.
Alternately, you can spray oven cleaner in it. Oven cleaner is as powerful chemical product (lye) in a spray can. The oven should not be hot when using it, maybe just a little warm. Follow the directions on the can. Some people imagine that baking soda will work like oven cleaner. I disagree.
Finally, you can clean it using ordinary detergent or dish soap. This requires a lot of scrubbing, and you must be careful not to scratch or break things inside the oven, such as the thermostat, the heating coils or the gas jets. You will also scrape up your knuckles and hands. Then there is the multiple rinses to clean out all the soap residue.
It may be that there is a pan at the bottom of the oven that can be removed and cleaned in your sink.
Personally I don't bother cleaning the oven. After using it for cooking a few times the grease will simply bake away.