Newbie Questions

There are a few things that I do not understand about WP. When I download a theme or plugin, I cannot figure out how to get the items into the Presentations area. I have the file extract folders on my hard drive, but cannot upload them into the site. Is there an upload function that I am not seeing?

I know this must be painfully obvious for most, so please be patient with me. I’ve read the training material provided in this site, but still run into the same problem. I’ve also tried to look for an area to create a new page template, but cannot find it. Do I have to overwrite one of the existing templates and save it, or am I missing this as well?

Thank you in advance for your patience.