Making the decision to sign up for a gym membership is a big deal for most. You are typically stepping outside of you comfort zone into an unfamiliar world that can be very intimidating.
You have no idea where to start so you automatically do 1 of 3 things:
1) you head straight for the cardio equipment
2) you sign up for a class OR
3) you stop going to the gym
So where do you start?
Most gyms offer a free session with a trainer to new members. Even if hiring a trainer isn't what you're interested in, he/she can at least show you around the intimidating gym equipment that you otherwise would have never tried. If the trainer is competent, they'll even show you how to properly use the equipment correctly.
This is a good start but what if you're gym doesn't offer a free session with a trainer? While options 1 & 2 aren't terrible choices, I would ask you what is your Purpose for the gym membership? What is your Goal?
Exercising at the gym without a specific goal is like driving around the country without a specific destination. You're going to end up somewhere, but it may not be where you want to be.
Begin with a long term goal. For example, if you want to loose weight, your goal may be: I want to loose 100 lbs this year.
Impossible right?
Once you have that long term goal, you can reverse engineer it make smaller much attainable goals. So that 100 lbs in a whole year will turn to 8.3 lbs a month, which turns into roughly 2 lbs a week!
Now you've take a goal that was overwhelming and seemingly unattainable and made it into something much more attractive and motivating! Now you have a destination. Now you can enter in the destination and follow the suggested route.
Start with a goal, then figure the rest out as you go!