What should i do for my dogs teeth? PLEASE help.?

I noticed while brushing my chihuahuas teeth that they didn't looked like they had some tarter build up. So i'm gonna start brushing them more often. but i noticed one tooth in the back, is kinda a greenish/brown color at the top of the tooth toward the gum. (the bottom of the tooth is still white) and the gums above it look dark red and irritated. I looked into the cost of getting it fixed and it's REALLY expensive. Money is really tight right now. So going to the doctors isn't really a good option for me. does anyone have any advice on what this may be and how i can help it get better or at least keep it under control? I bought teeth cleaning treats and i now brush his teeth everyday. But i still don't really know what is going on with that tooth. Thanks