You can't outrun a bad diet. In other words, no matter how much you exercise, if you consume more calories that you burn, you're going to gain weight.
Start by calculating your total daily energy expenditure (TDEE). That's the total number of calories your body requires to maintain its current weight. Google "TDEE calculator" and several options will come up.
Next, determine your caloric deficit. A deficit of 3,500 calories will result in one pound of weight loss, so a 500 calorie a day deficit will result in one pound lost per week (7 x 500=3,500). I recommend a deficit of no more than 10-20% of your TDEE.
Finally, track your calories. If you're honest about everything you put in your mouth each day, you'd be surprised at how quickly it adds up. I use Myfitness pal - it's got the nutrition info for just about everything in its database and is easy to use.
Good luck!