Going to have to be brave and lift that lid! If you can come back with what's underneath, (the chamber construction, NOT the waste....), then may be able to advise you further. There may only be one cover, construction methods have changed over the years. If no apparent outlet into a ditch or stream then its likely the outlet goes onto some sort of soakaway, to allow the final effluent to disperse into the subsoil. Various regulations regarding this, more info on the Environment Agency website.
Be warned, every drop of water you put down the drain will find it's way into the tank. It relies totally on the soakaway to continue functioning, but if the water table is high or the soil conditions are not very porous, the tank will soon start to struggle and effluent will back up.
Assuming no problems, it also pays to remember septic tanks rely on bacteria to break down the waste, treat it with respect, as said keep use of bleach and household chemicals to a minimum. Anything other than human waste and tissue will not break down, and fat/grease will congeal which can lead to blocked pipework.
May be wise to get it desludged before starting to use it, then get it desludged on a regular basis (how often depends on size of tank and how many are using it), allowing it to get too sludged up can result in a high concentration of suspended solids going down the outlet which can cause the soakaway to bung up....