my landlord will not fix the septic tank and there is sewage all over the yard. can i withhold my rent till they fix it?? what are the eviction laws??

I cannot answer because these are state and local law specific, but generally, you need to provide the landlord notice and a reasonable time to fix the problem. If it is not fixed, ask to break the lease with no penalty and move immediately and ask for your deposit back, since this is an instance in which the landlord inability to deal with the problem is critical and makes it impossible for you to live there safely.
You can call local public health for help, but they will likely tell you to move immediately, and that sounds like that would be best, but if you ask and do not receive reasonable service to deal with the problem, you should be able to get your deposit back easier and if you need to go to court to get the deposit back, your case is stronger.
Consult local counsel as they can give you a better read about how these matters are dealt with where you are at. Most attorneys charge nothing for initial consutlation.