Help needs to stay on the forum so all can benefit. I strongly recommend that you remove your phone number---unless you are ready to be contacted by every tele-marketer on the planet.
You can download the CD images from the Gentoo website. Simply burn the CD(s) from the images, and then boot from the first one and follow the instructions.
If you have been away from Linux for awhile, there are other distros you might want to consider. Take a look at http://distrowatch.com
If you need to get into the computer as it is, try booting into "single-user" mode:
When you see the GRUB menu, hit any key to stop the count, (escape if necessary to get out of graphical mode), then "e" for edit.
Select the kernel line, "e" to edit
Add "single" to the end of the line
You will now be in a root login with no password required. You can then change the password by entering "passwd root" or "passwd <username>" You can then boot up and log in normally.