Need some help with diagnosing my car. Young guy learning

hey guys. im new here, i see around the thread alot of helping going on and was hoping you guys could help me with my first car. i have a 75 duster and its been acting up since i got it. there would be days when it simply wouldnt crank and accessories like radio and lights wouldnt work then they suddenly work again. that was a couple months ago now something similar has happened and it hasnt been fixed for days. ive replaced the starter relay and the fuses, and made sure the battery was good. tested with a new one but still nothing. hope you can help out. Any thing is much appreciated