Joining the walls of a shed to each other

by Jim
(California)

Hello! My son and I are planning on building an 18'foot x 18'foot lean-to style storage shed. When I was a kid building walls with my own dad (about 60 years ago), I did all the hammering and my dad did all the plans, so I never knew how to lay things out.

On the shed we are going to build, I want the studs on 16 inch centers, which I know how to do, so building the walls isn't an issue. But I don't remember how to join the walls of a shed?? I know that the 2nd/top plate (on 2 of the opposite walls) will over-lap the shorter adjacent walls, but which walls are to be built a bit shorter (to go inside the longer walls), or does it matter?

I am assuming that 2 of the shed walls will be shorter than 18' foot long, to fit in-between the two walls that ARE 18' feet long, but which walls should be the longer ones? The front and back walls? Or the two side walls?

Also, I'm not to sure how to frame the roof on a lean-to shed, but I know I don't want that much of a slope (we're in southern California so we don't have a snow load or hurricanes to worry about - just occasional rain storms)

Can you help?