How do key signatures effect music?

it does not

After the clef sign, before the time signature.

Basically you look at the sharps and flats that are near the treble/bass clef, and use that to tell it. There are more complex ways to find major and minor, but minor tends to be sadder, and I don't...

Always, unless you're writing atonal stuff, or just writing it in C and adding accidentals to everything, or if you're writing in the dreaded 12 tone..

The naturals simply indicate that the five notes that were previously sharp, are no longer sharp.The key will be C Major, or a minor.