Welcome Jennifer!
First of all, take a deep breath
Yes the site is big, there is lots of info, and everyone has been where you are at one time, so don't be afraid to ask questions. But take your time, you won't learn everything in one day.
As for your layouts....
I save a copy of my layout in.psd format, in case I want to make any changes later.
For uploading to the web, I flatten the image, then resize to 500x500 pixels and 72 resolution. Then I choose File/Save for Web. This will save as a compressed .jpg. You can fiddle around with how much to compress. The file needs to be 125K or smaller to upload to SBB.
And yes, this is not the file you would want to print from. For printing you probably want 300 dpi.
hope that helps a little