Start small. Don't try to launch into a whole ton at once. I started out with 3 days a week, half an hour at a time, and built up gradually as I became accustomed to it.
What you end up doing should be based on what your first goals are. If you're trying to get rid of the beer belly, you'll probably want to put more of your initial effort into cardio than weights--it's best to do some of both, but the cardio burns more calories in a given amount of time, and the added caloric burn from gaining muscle is actually quite small. If your feeling out of shape is more that your arms threated to give out when you're hauling in your groceries, you'll probably care a bit more about the weights. If it's a more generalized lack of energy, you'd want a more even split; if it's that you get winded from 3 flights of stairs, you're going to want to do more in the cardio department.
Ask for help. When you don't know what you're doing, ask the gym staff, or someone who looks like (s)he is moving the weights slowly, fluidly, and in complete control. Lighter weights handled properly are both more effective and safer than large weights used incorrectly.
Realize that it doesn't matter if you're using lighter weights, or doing fewer repetitions, or whatever, than anyone else there. 1) No one is likely to notice; and 2) even if they do, they were probably in that boat once too.
Accept that results do not come immediately. Ignore your scale, and pay attention to whether your clothes fit, whether you wake up with more energy than you used to, whether you sleep better now, whether those stairs are easier to handle now, etc.
Don't make anything all-or-nothing. That's a plan for failure. There will be days you don't end up going to the gym, for whatever reason. there will be days you decide to eat a whole pizza. Recognize that these will happen, try to limit them, and then get back on the horse.