You need: A friend with a Macintosh, the Snow Leopard DVD, and any external drive. Could be a USB hard drive, could even be a USB flash drive (8 GB at least).
On your friends Mac, plugin your external drive. Start "Disk Utility", it is inside Applications in the Utilities folder. In "Disk Utility", click on the external drive, click on "Partition", and change it to one partition in "Mac OS Extended (Journaled)" format. There is some option to choose to turn it into a bootable disk, but the software will tell you.
Insert the Snow Leopard DVD. In Disk Utility, click on "Restore". Drag the Snow Leopard DVD onto "Source", and the external drive onto "Destination", then click on "Restore". A few minutes later you have a bootable USB drive.
Try that you can boot from it: Restart the Macintosh, and hold the Alt/Option key pressed. The Macintosh gives you a choice which device to use for booting, select your external drive. When it boots, you should now be running the Snow Leopard installer. If that worked, then take it home.
Didn't realise you had things on your Mac that you need to keep: Buy an external hard drive. You really, really, really need one - if you had bought one earlier and used Time Machine, you wouldn't be in this trouble. Again, plug it into your friends computer, use Disk Utility to change it to "Mac OS Extended (Journaled)", then restart from the Snow Leopard DVD and _install_ Snow Leopard on your external drive. Again, try that you can boot from the external drive (hold the Alt/Option key down while restarting). If that worked, do the same thing at home. If you aren't completely out of look, you should be able to boot from the external drive, and both drives should be visible. Copy everything that needs copying to the external drive.