Download 'Option 1' from here:
http://silverstripe.org/stable-download
Extract the files from the archive. Windows default .zip program won't work for this archive type, you'll need another program like jZip
Log into your account's FTP.
Go to public_html
If you want your root (domain.xom/ instead of domain.com/directory/) to be the silverstripe, skip this step:
Create a folder/directory inside of public_html and go to this directory
Upload the files you downloaded to the currently open directory
Access the directory you installed to using a web browser, i.e. Internet Explorer, Mozilla FireFox, or Google Chrome
Follow the instructions to finish the installation.