How to deal with mistreatment at work.?

I have worked in a health care office for a year now. At first everything was great. Then recently my boss has been treating really unfair there was an incident where a co- worker of mine threw some very important documents in the shred instead of filing in the correct spot this is not the first time she has done things of this nature to avoid getting out of work she cuts alot of corners to get of doing things the correct way. This person was working in the clinic I usually run. I found the documents when I came to work the next day. I immediately took the documents to my bosses and told them I had found the papers in the shred bag. Then because the person who did this is friends with my boss they tried to turn it around on me and say that I put them there myself just to get this person in trouble. Now they have moved me to the clinic that this person works at full time and put me in a position were I have to work with this person everyday. Now the work environment is full of tension and to top it all off i'm not even supposed to be at this particular clinic because it has stairs and I have a preexisting knee injury and a doctors note that I gave to my bosses showing them that i'm not supposed to be doing the stairs they just ignored this note. I do a good job and it doesn't even get noticed. Now I have been talked to and am on the verge of losing my job for something that I didn't do. How can I fix this situation and make my voice be heard?