Download an app like MyFitnessPal or some similar. When you set it up, you have the option of choosing whether you want to lose weight, maintain your weight, or gain weight. It asks you information like your current weight, height, activity level, etc.
If you simply want to not gain any weight back, choose the maintain your weight option. It will tell you the amount of calories you can consume each day. Whatever exercise you do, you can log that as well. If you follow this system and you're as accurate as you can be with your measurements and whatnot, this will really help you.
I would suggest buying a food scale like
this one. You'll need that to get the most out of the fitness app.
Something to remember...exercise is probably the best thing you can do for your overall health, but let it take a backseat if controlling your weight is your primary concern. What I'm trying to say is: don't think you can eat whatever you want and make up for it on the treadmill. Be disciplined with your food choices and control your portions. Stay within the parameters the app gives you and you will succeed.