No worries, I'm used to dealing with non-techies, haha. At least you're trying to do stuff on your own!
For the installs, you could do it either way. If all of the installs are located on the same server, it'd be easy to have them all in one MySQL db with different prefixes on each install. Or, you could create a unique db and username/pass for each, to keep things separate. To install the Gallery3 script, you need to upload the files you download from their site to the base directory of the website (/public_html) or download and untar it with wget and tar in a linux environment and place the files in the /home/domain/public_html or comparable folder for that site/install.