my dog wets itself when i combs its fur

 Some details, history? I'm assuming the dog is urinating when you try to groom it. Is the dog a puppy? A boy or a girl? Neutered? Have you had it for a long time?

Its a girl, about 3, i got it from the rspca, it was beaten up as a puppy. i've had it about 6 months.

 Do you know that she was beaten? What kind of dog is she? Is her hair in good condition? Where are you when you try to brush her? Where is she? How is she at the groomer's?

My first thought is make sure that your dog doesn't have any lumps that might be causing pain. 

If not, bring out the brush/comb and don't do anything except give a special treat but plan the event.  If your dog is small...pick her up and cuddle then slowly (carefully) expose the grooming tool on a counter top and treat.  Then remove your dog from the area  and treat again so that she is not thinking hmmmm...this is trick situation...and continue to cuddle. 

If your dog is larger and can't be picked up and cuddled, then make sure she can't make a run for it...make the area small....bring out the grooming tool ... and treat.  You can gently place the tool on the floor and then gently toss a treat close by.  If, with time, your pup goes for the treat....you will soon be ready to move on to the next step.  It will also help to make yourself small...so get down on the floor with your dog.

The fact that your dog is wetting itself when combed is a very strong message...so take your time.  This is a trust building thing...put a lot energy into it.   In fact, though...if you repeat the exercise slowly and gently, it really shouldn't take too long before your dog thinks that the grooming tool is okay.  Then the next steps take place.

When you first try to comb again, just do a few light strokes and treat.  Then do the happy thing and get back at it the next day.

I had to do this when I first cut my (rescued) Rottie's nails.  He was two and a half when I got him and it was kind of scary for both of us....so I did a nail at a time and worked my way up to a paw at a time etc.

Good luck...

It could be that your dog is really fearful about being brushed and combed, and the wetting is "submissive urination."  Dogs do this when they want to tell you that they are not a threat - it's a way of defusing a situation.  One thing you could try is to slather a bit of peanut butter or cheese whiz on a flat surface, and let her lick it while you brush her.  She will start gradually to associate being brushed with getting a good treat and, hopefully, won't be so anxious about it.  Be gentle at first, and careful not to pull any tangles.  Right now, you want every experience she has with the comb/brush to be a positive one.