Whatever weight I can just do 10-15 reps with, that's my warm-up set.
Whatever weight I can just do 6-10 reps with, those are my work sets.
Some people prefer higher, like 20 warm-up and 12 work; some lower, like 10 warmup and 5 work.
I like a short but intense workout, so I do 1x warmup set, and 2-3x work sets of each exercise. Others prefer a longer workout, or they focus on one bodypart for that session and do the others in other sessions; so they have 2-4 warmup sets and 2-4 work sets.
Generally speaking, higher weight and lower reps builds strength and size, and lower weight and higher reps builds fitness. But if you don't push yourself, you won't build anything. That said, in the first few months, just focus on getting into a rhythm of regular exercise - nothing works if you don't do it regularly! - and learning the movements of the machines and weights or your chosen sport. It's better to lift 10kg with good form than 30kg sloppily.