Will a brace be able to fix a crooked smile or could it be something to do with my jaw?

Possibly: Symptoms like these are best deterimed by a thorough evaluation by your physician. Based on his/her findings, a treatment plan can be developed to help you.

See Orthodontist: Malposed teeth and or jaws can cause an uneven smile as can neuromuscular problems. Have your md/do do a neurological screening, but make sure you are examined by a qualified orthodontic specialist.