If you've been active and you can't figure out why you're not losing weight it's probably because you are eating more than you realize. It's easy to mindlessly munch. People who track what they eat have been found to lose more weight than those who don't. A notepad will do the trick, but you have to look up the calories every time and add them up and calculate what you should be eating.
Now there are tons of websites that do all the work for you.
I use http://www.livestrong.com/. I like the workout tips and bulletin boards on livestrong's website. It tracks all my calories and remembers my favorite foods, so I don;t have to re-enter my favorite meals over and over. It has a database of almost any food you can think of including restaurant menu foods. It also tracks my exercise and subtracts the calories I burn from the ones I've consumed.
Alyssa Sugar the therapist I spoke with for my Losing It series uses myfitbuddy.com. It has a social media aspect so you can chat with friends about your progress or share recipes.
Here are some other websites to choose from to make your food journal as easy as facebooking. Per trainer, Lose It!, and Sparkpeople.
Most of the food journal sites are available as apps too, so you can have them on your phone. You want convenience- you got it!