This thread has lots of helpful information:http://www.thecatsite.com/forums/showthread.php?t=9563
First, as Helen suggested, take her to the vet and let the vet know about the peeing issues, so she can test for urinary problems. If she has an undetected health issue, nothing you can do as far as behavioral treatments will help until the health issue is resolved.
Is she spayed? If not, spaying her would be a good idea.
You'll also need to clean the mattress thoroughly with an enzyme cleaner (as suggested in the thread above) to remove any residual urine odor.
If she gets a clean bill of health at the vet, I'd suggest putting a waterproof mattress cover over your mattress to prevent any further damage to your mattress. You might also want to cover your bed with a tarp or thick sheet of plastic to protect your bedding.
My cat Felixia used to pee on the bed. I used a tarp to protect the bed while trying to figure out what was wrong, and while the vet ruled out health issues.
In her case the problem turned out to be a combination of dissatisfaction with her litterbox, because I was working long hours and she's very picky about having a spotless box, and stress, because I was gone so much she wasn't getting very much personal attention.
What fixed the problem for her was adding an extra litterbox and using cat attract litter, using Feliway
(I used the plug-in
for my apartment, and used the spray
directly on the bed), and dedicating some time each day to interact just with her, away from the other cats.