I’m working on a volcano scene and wanted to add some trees and may be a few buildings, but whenever I spawn a cylinder it always comes up diagonal to the flat ground, how can I rotate it so that I can begin the tree trunk?
when creating objects they always appear in the same angle in relation to your view. To create the cylinder so it’s the right way from start, you sould be in top view. To rotate and object, selct it and press ‘r’
Yeah I figured out that whole top view thing shortly after, thanks for telling my how to rotate it though. 
You might want to read the wiki and learn about all the other options you have, like scaling, and parenting, constraints.
Try Alt+R at the beginning