Shutter speed intervals:
Do the math:
F8 at 1/125th should be equivalent to F5.6 at 1/250 (or F4 at 1/500th, etc) in terms of achieving the same exposure.
F/11 at 1/8 of a second has the same exposure as 1/250 at f/2. It's just math, you open up the aperture you are adding light, adding 1 stop means if you want to keep the same exposure you can increase the shutter by one interval.
These reciprocals are important to know and understand in photography. The variable would be your ISO but for now let's assume you are keeping the ISO constant.
Here's a handy chart someone made up. You can download it, print it, whatever. But the important thing is to get to know it because being able to know these numbers will give you greater control and increase your possibilities. It will also help you with lighting and this will be covered shortly.