When to use what in an automatic transmission?

My extent of car work know how is checking the fluids, tire pressure & adding where needed. That’s about it & I hope someone can answer this question for me. I just got an 1988 Chevy Suburban R10 (actually it’s MY 1st car). Just for a bit of background on the question I’ll give you all the gears it has P, R,N,OD,D,1,2. My question is when do I use what gear? I live in Montana & have used the 2 & 1 on ice & snow to help with both warming up engine & also those gears seem to help with traction & slowing down on slick roads. My main question is when to use OD verses regular D but would also be helpful on when to use the 2 & 1 as well. TIA