Basically what Sara_Numbers said. I'm not going to lie, it's a total pain in the ass starting out. I know I started and stopped a couple times because I hated it/couldn't be bothered with it. But it gets easier with practice and now it's no big deal. Based on my own experience, here's what I'd recommend:
1. Download app
2. Spend a couple weeks logging what you eat after every meal, but don't worry about the amount. What you want to do is build up the habit of logging your diet.
3. Once that becomes no big deal, you need to start paying closer attention to the quantity of what you're eating. This part can be really hard, especially if you don't cook for yourself. Ideally you'll have a food scale to measure amounts. Most people don't want to bother with it, but it's the best way to keep track. On the plus side there's a barcode scanner for prepackaged food, and if you mostly eat the same foods (and the same amount of foods) you can get away with measuring things only a couple times. Once it's in your history you can basically copy/paste it.
At this point you'll probably either say "fuck this, I'm not doing that" or you'll try it and say "fuck this sucks, I quit." I know I did. If you do, don't worry about it, but try again later. Each time I started to do it again, the further along I got (and eventually got a food scale) and got the hang of it.
Once I more or less got that part down, I used an online calculator to find my TDEE (maintenance calorie level) and aimed for 300 calories below that (this is variable depending on how aggressively you want to lose weight). I think LoseIt! and MyFitnessPal has that integrated into the app. I also eventually found this
TDEE estimator which I think is pretty neat. You put in your weight and total calories each day, and after a couple weeks it's got your TDEE down, and dynamically changes over time as needed. It's not strictly necessary, but it works well and takes out some of the guesswork.