I'm 18 and i have a underbite is there any way to fix it without getting surgery? i really don't want to get surgery

Yes: Yes, often can be corrected with braces. HOWEVER, often there is a compromise in cosmetic appearance. Make sure you are advised of ALL your OPTIONS by a BOARD CERTIFIED ORTHODONTIST. Get a SECOND OPINION from a BOARD CERTIFIED ORAL SURGEON who does that type of surgery.

Degree: Depends on the degree of skeletal imbalance. Mild under bites may be camouflaged by tooth movement alone. More pronounced under bites will require both tooth movement and jaw repositioning, or the parts won't fit. See well qualified Orthodontic Specialist and Oral Surgeon for evaluations. Remember, if it sounds too good to be true.....

Possibly: Really depends on the amount of "underbite". In most cases orthodontics can provide an improvement. Speak to an orthodontist/oral surgeon team who can evaluate you and advise you of your options. All the best...

Depends on severity: Underbites go from A-Z in severity. Mild under bites can be corrected orthodontically by changing inclination of anterior teeth. Moderate underbites can be corrected orthodontically by removing two lower bicuspids and retracting lower front teeth. Sever underbites generally require a combined orthodontic/surgical treatment. Seek a consult to see what your options are.