I have a bad broken tooth (molar). only half of the back isleft. what should i do?

Prompt dental eval: This really needs assessment by a dentist - please don't put off. If you are in significant pain it may need to be evaluated promptly.

Sometimes not: If a tooth is severely decayed below the socket or cracked down the center, or its roots have decayed into two or three separate pieces, you will need it extracted. Teeth decayed to the level of the gums or ones with a simple fracture may be salvageable. Ask your dentist about long term success rates. Having an extraction and an implant may be more successful /less $$ than "hero-dontics!".