You need to go in with at least that. Whether it's to lose 20lbs, to become stronger, to make the football team. Whatever it is, it needs to be for YOU, and it needs to be realistic. It can't be 'to lose 50lbs this month'...that's is probably not going to happen. So what is it? How big is it? Can it be achieved in 3months? Once you figure that out, and what it means to you. You're already half way there!
So now that you have your goal, what's next? MEET WITH A TRAINER. Your gym should offer you a few sessions complimentary with your membership. Take advantage of these. Read the trainer bio boards, pick someone you think will fit. Whether it be his/her look, education, fitness philosophy, etc. The trainer will set you in the right direction, whether that's a jump start program or you become a client. Everyone needs a trainer at some point. They will tell you what it's going to take to get to your goal. So don't be afraid to sit down with a trainer. After taking initial measurements and going over a medical history, they should get into program design (workout frequency, SAFETY, resistance training, cardiovascular training, correct loading, training split, etc). So it is literally impossible to not be set in the right direction after speaking with a personal trainer. You will know if he/she is just looking to sell you or if they actually have compassion for your situation. Maybe the trainer bio boards didn't do it for you and you can't decide from that alone. Take your first week to watch the trainers and get acclimated to the facility. Watch how each trainer interacts with his/her client, watch how he/she trains, and make your decision from that. I promise you won't be sorry.