dose anyone know how to make a riff that is in the right key

im not sure how much theory u know. so ill tell u an easy way to get on track.
if its just an easy riff, not a huge solo...

if u know the chord progression of the song.
example; G,C,Am,D...

use ur electric tuner to find these 4 notes on the fretboard(or use a fretboard chart.) make sure they are groupd together.

try maybe around the 12th fret, or actually anyplace on fretboard u like.

then, during the song, follow along with the progression, using these 4 notes. until it fits how u like. add other notes inbetween for fill.