How can i make an exercise program to suit myself?

im trying to lift weights, i was muscle, i dont want to be a bodybuilding champion or anything, i know alot about diet already, so thats ok, but im still having a hard time make an exercise schedule to suit myself, like how much weight do i lift? i lifted 60lbs 60 times doing 15 reps each time, having 30 seconds and then going again, but my arms have been sore for days now, i think it was a bit too much, but then i need to struggle with around 15 reps and lifting 60lbs didnt seem like a struggle, even at 60, but i feel it now lol, and my dad said do not lift too heavy because he knows a friend who tore his chest muscle or something, or shoulder muscle n now its affected it forever, so my question is

ive been lifting weights now and then, but now i wanna do it often i hear people say your meant to struggle on the last rep you do, but im a bit nervous to do that because of what happened to his friend, i wouldnt want that to happen to me... any ideas?

i just read this back, and it sounds like im saying im 60 years old lol, when i say "even at 60" i mean even lifting the weights 60 times