Re:Reps and weight?
Pick a rep range, i.e. a range that you would be happy with for your purposes, for instance 12 to 15 reps.
Then, after any necessary warm ups, go for a weight that would only just allow you to get to the top of your rep range on your first set. then on the second set use the same weight and go all out as many reps as you can, this is likely to be just short of the reps on the first set for example 13 or 14 reps. Then use the same weight again and go all out again, this time you might get 12 or 13 reps. Sets done.
Stick with this weight until you are getting too many reps on your third set for instance 16, 15, 14, then go up by the smallest increment that you can. This might be adding a 1kg dumbbell to the top of the weight stack and continue as before.
Repeat until you are using the entire stack and then move gyms.