Hey @Harper_Harris welcome to the forum!
Whatever key you’re playing guitar in is just a half-step lower.
Playing a G chord? You’re playing F#.
Playing an E chord? You’re playing Eb.
So taking those 2 keys as examples, to play 2nd position you’d need a B harmonica, and an Eb harmonica respectively.
Regarding minor keys, the Natural Minor harps are nice cos you can play chords. Lee Oskars are labeled IN 2ND POSITION, so you just order the key you need. For example, if You’re playing in Am on your guitar (½ step tuned down) then you’d order a Lee Oskar Abm Natural Minor harmonica. If you’re playing in Em, you’d order an Ebm, et cetera. Here’s a little more on Natural Minor harps: Natural Minor Will Explode Your Harp Game (+ Natural Mystic on Am w/Tabs)
Regarding figuring out 2nd position keys, this might help:
But if you have any specific questions, just post 'em here and we’ll help ya out!
Rock on! 