If you are going to go to the gym. Spend the money for 1 or 2 sessions with a personal trainer. Tell him your honest goals. Let him know how much time you have to train, and how many days a week you will be in the gym. Have him set up your routines, and show you the proper form to use.
Set up logs, and chart your progress. You will not remember every routine, how much weight you are using, and how many reps you are doing. Write it down.
You don't need a personal trainer every workout, but every few months, you should schedule a session with the trainer. He can make sure you are still following proper form, and show you new exercizes that you can do to keep your workouts "fresh" and fun.