Okay first of all, the logging system on fitbit is not very good. Like at all. So it really doesn't matter what you put into it, it's probably not going to be all that accurate.
For me, I've been counting calories since I was fifteen and I know what food portions look like, and I'm pretty good at guessing about how many calories are in something that I eat. Unless it has a really high sugar content, that'll usually throw me off a bit.
What I'll usually do is just plug in exactly what I ate into the logger. If the thing that I ate doesn't show up in the dropdown menu, I'll just type something similar into the box and then select an option that has approximately the amount of calories that I think I ate. I realize this advice is probably not helpful for you now, but if you ever get to the point where you can mentally visualize 3/4 of a cup of skim milk, it might.
Tl;dr: Honestly, just calculate your calories on some other site, like calorieking or myfitnesspal first. For spontaneous meals that are homemade, try and find a store bought equivalent, and guess at your portion size. Always overestimate if you're trying to lose weight and underestimate if you're trying to gain. It'll give you some wiggle room to work with if it wasn't your last meal of the day.
