I apologize if you already know this, but if you don't, there's a very important relationship in exposure you need to memorize/understand. f/stop is the aperture size, each "whole" stop on the f/stop scale lets in half or double as much light as the stop up/down.
So, f/2.8 let's in half as much light as f/2.0, 4.0 is half of 2.8, 5.6 is half of 4.0, 8.0 is half of 5.6 and 11 is half of 8.
Every time you cut your aperture in half, you have to halve your shutter speed or double your ISO to get the same exposure. So if you are at f/4 at ISO 400 and shutter speed 1/100 and you move to f/5.6, you have to get another stop out of your shutter speed or your ISO to keep the same exposure. So, you either have to go to ISO 800 or ss 1/50 to keep the same exposure.
The best thing I did for myself was write down the f/stop scale and then kept it as a reference wherever I went. The full stop scale of most common f/stops (from biggest opening to the smallest) is: 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22. There are also half and third stops in between, but these are the most common whole stops.
Again, apologies if you already knew this, but if you didn't, it's a critical relationship to remember.