Ditto that website. Krista has some good advice.

Do NOT lift to failure. Your last rep (lift) should be difficult but the form should still be excellent and you should feel like you could have lifted once or twice more. If you get sloppy, you can get hurt. Remember that muscles build faster than tendons & ligaments. People will often get joint problems because they are lifting too heavy in the beginning.

If they want high reps, that usually means 15. I personally lift like that only on my light, recovery days. Other than that, I lift between 6 - 12 reps depending on the day. For example, one day I might lift 6 reps and 4 sets, the next day 10 reps and 5 sets, etc. At 15 reps, I'd probably lift 2-3 sets. I pre-plan all of this, so I know exactly what I'm doing when I hit the gym.