Does melanoma get caused by sunburn or is it something else? im 25 how likely is it i have it ...got a flat mole in my beard...dont go outside much

Sun and genetics: Exposure to UV light and genetics determine the occurrence of melanoma. You may visit this site for information on this topic: https://www.skincancer.org/skin-cancer-information/melanoma

Keep a good watch: Everybody is at risk for melanoma. Some folks at more, some at less, but everyone needs to keep an eye on their skin, have someone else look where they can't see, and have all suspicious lesions seen by someone who knows their stuff. "How likely?" is the wrong question here, as I think you realize. Just assume you're at high risk and act accordingly.