If i put water and salt in a cup and put it in my mouth will that help my swollen gum?

Won't hurt: But probably won't help much. You need do see a Dentist, or preferably a gum/bone specialist to diagnose the cause of the swelling (most likely, but not only cause, is infection). Once cause has been diagnosed the correct treatment can be applied. You need help. See a professional.

Not particularly: Why is the gum swollen? Abscess? Inflammation? Erupting tooth? Why not see a dentist to find out.