When i'm just kissing my girlfriend my pants get semen on them, will i get use to this? i'm 18yrs old and my first girlfriend.

Premature ejaculatio: Yes , your status will improve.

Congratulations: When you get excited like this, you're certain to produce some lubricant ('pre-cum') and you may ejaculate. As you gain experience, you'll get some control over this. To keep intercourse lasting longer, a dab of a topical anesthetic or a condom may slow you down, or you can learn the squeeze technique. Your physician may rx a low dose of a ssri but i'd advise simply learning / practice.