When I was in middle school, i was very small. I was maybe 115lbs with zero muscle. I decided after 8th grade graduation that I would start picking up my dad's 15lb weights and transform my body. So after everyone went to bed i started doing pushups, situps, deltoid laterals, bicep curls and tricep extentions. when I started out with my biceps, I could barely get 30 reps in with the small 15lbs. every single night of working out I improved. I eventually got up to 100 reps every night with both bicep curls, tricep extensions and the deltoid laterals. With the bicep curls, I was able to do 205 reps until my wrists couldnt take it anymore. But thats how I started and I havent stopped since. Within the last year was the first time I started using supplements or having a spotter. therefore I've gotten comfortable lifting higher weights whether I have a spotter or not. For the most part I now know how many reps I'm going to be able to do with certain weight.