I can remember my father (a choirmaster from my childhood) who would put the church choir in key before attempting a piece of music by having us sing the scale, then having us sing the interval from the root to the starting note. He could then demonstrate that we could learn to reverse the procedure by singing something new and then find the related scale. This became quite easy with major scales and some of us caught on to minor scales the same way too. Naming the key of the scale was immaterial to the choir, but comparing the root of the scale sung to a note on an instrument was teachable to kids. Making it a friendly game for kids made it fun.

An hour with a decent teacher of your instrument and a notebook would answer all your questions as to what key you are using, and it would be worth the expense. Failing that, find a chord reference organised by key and a book called "The Mini Guitar Grimoire" by Adam Kadmon, or "Scales And Modes In The Beginning" by Ron Middlebrook.

Honestly, fifty bucks or so will pay off bigtime.

Peace, Mooh.