Cutlass Supreme,my battery kept draining until i bought a new alternator and battery. i have tightened the belt on the alternator but it still die

My cars battery was always draining until i was told that it could use a new alternator. I bought a new alternator and a new battery. I tightened the belt on the alternator as much as i could (i dont want it to snap) but the same situation still occurs. Within 2 days my battery is fully drained. Some tell me that it could be a wiring issue, others tell me it could be the starter. Please give me some hope and let me know if you can help me out to fix my situation on my car. Thank you
My car has an automatic transmission.