For definition you should choose a weight that allows you to do 10 - 15 reps. If you can't do 10, it's too heavy, if you can do 15, then increase the weight by 5%. Try to do one more rep or increase the weight every workout.
If you are just starting out again, go for one set for the first 4 weeks and then if time permits increase to two sets the second month and three the third month.
You can get significant results with one set, but it must be an intense set, i.e., moving slow and controlled with no momentum.