The Circle of fifths diagram below is the heart of basic music theory, and is possibly the most important section on this site to read after understanding how major and (natural minor) scales are constructed.
Having learned how to draw it from memory using a few simple memorable phrases, you will automatically know how many sharps and flats are in all popular scales without working them out by hand.
You will also know all the relative major / minor scales, which scales contain identical note pitches, but have different note names, and which scales sound most alike and work well together.
The Circle of 5ths sections seem to be long, and there appears to be a lot of material to get through, but there is a lot of repetition once you see the patterns, then you will fly through it.
Suggest printing out or saving the above diagram on your desktop for reference before you start.
Get started now with Circle of 5ths first section - 1. Introduction.