Finding the right weight for you is pretty much trial and error.
It's going to be different for every machine and every exercise you do - that's why it's really helpful to carry a notebook around with you in the beginning to keep track of your weights.
Generally speaking, when you are starting out, you want to be in the 12 - 15 rep range. That means you want to be lifting a weight that becomes challenging or almost impossible to do another rep somewhere between 12 and 15 reps. If you get to 15 and you can keep going, you know to pick a higher weight for the next set. If you can barely crank out 8 reps, you know to go lower the next time.
Just as a guideline, when I do an orientation in the gym, I typically would start with these weights - you could try starting here and see how you do:
Lat pulldowns - 30#
Seated row - 30#
Pec flye machine - 15#
Chest press machine - 30#
Shoulder press machine - 20#
Bicep curl machine - 20#
Arm extension machine - 20#
Cable pushdowns - 20#
DB curls - 8s
Kickbacks and other tri exercises w/DBs - 5s
DB shoulder exercises - 5s
Have fun lifting - you're going to
the results!!