start with having your septic tank pumped out. Depending upon its size, it may need it every 1 to 3 or more years. They use a big vacuum truck to empty your tank. Look in the yellow pages.
Your water logged drain field is not allowing any effluent to soak into the ground. Hopefully your drainfield is not completely saturated where you and your neighbor would need to put in a new field on another part of your property.