Pita, that sounds like a great program! I like an upper-lower split too, but you may find six days to be too much. It would be OK to drop back to a five day split of three UB days and two LB or even four days of alternating upper and lower. I would personally have a hard time recovering from three leg days a week.
Since you're just starting, aim for the 12 - 15 rep range. Pick a weight that becomes challenging, if not almost impossible, somewhere between 12 and 15 reps. If you get to 15 and can keep going, then the weight is too light and you need to pick a heavier weight for the next set. But, if you fail before 12 reps, then the weight is too heavy and you want to go lighter on the next set.
You say you couldn't make it to 15 -- did you make it to 12? If so, I'd say stick with that weight and just do what you can so long as it's in the 12 - 15 range. Once you can get to 15 fairly easily (and it will happen soon!), then go up in weight.
Happy lifting!
