Track your calories. This is really easy using the free app, MyFitnessPal. Even just doing it for a week will help you understand the calories of the foods you eat and the target you need to hit.
Trying to gain weight? Set a higher daily target.
Trying to lose weight? Set a lower daily target.
I know some people who also weigh themselves daily with something that automatically charts their progress over time (my dad adorably does this using the Wii Fit; I have a fancy digital scale that sends my stats to my phone â I get a lot of value out of seeing trendlines without actually having it in my attention for more than what it takes to step onto the scale at the same point in my routine every day). I also like that my scale makes an attempt at judging my percentage bodyfat.
See also Fitness Blender and other things under Exercise.