2) Just make sure that you always have a plan going into the gym. What you're working that day, how many sets you're doing of each.....etc (Chest day, leg day...whatever your split is day). One thing I've observed that can cause people to stop going, are guys running around to all different types of machines, no plan, then 3 months down the road (if they make it that long) nothing really to show for it. All because they didn't plan out what they were doing or what their goal was.
I want to add a little to this. I agree that you need a plan, but it's a little bit different for someone who's just starting out.
If you've never lifted weights before, don't go in with a 5 day split hitting one particular muscle group for an hour and a half every time. This will kill you with soreness and you'll probably want to give up right away. When your muscles aren't used to working hard, they get sore much more easily, and need a bit of time to get stronger and be less prone to soreness, so that you can then really push it hard and get that "hurts so good" soreness.
What I would recommend for a beginner is start with something like a 3 day split, doing only 2-3 sets for only 2 exercises per muscle group, with lighter weight. It's all you need to be hurting the next day for now, and it's very important to get your form down right early before you start going to the heavy weights. I see so many people who have no idea how to do a squat or a deadlift, and that can cause some serious damage down the road.
A pretty good beginner's workout could be (although not necessarily) something like:
Monday - Back and biceps
Tuesday - off
Wednesday - Chest/shoulders and triceps
Thursday - off
Friday - Legs and core
And once you're not getting sore anymore, that's when you need to start going heavier AS LONG AS YOU HAVE PROPER FORM. And start working out longer, more different exercises, adjusting your split, fewer days off, etc
Anyway, it seems like most of you replying here have had some experience in the gym but I figured there might be others who want to get started but aren't really sure how.