The 1-800-MYAPPLE store will sell you the 10.6 installation CD. If your data is backed up, just simply boot the installer disc, and run disk utility to reformat the hard drive. Then install 10.6 from scratch, and restore your backup.
If your data is not backed up, use a data recovery tool mentioned in my tip 1689 above to recover the important data to an external drive, before erasing the internal drive, and installing 10.6 from scratch.
Once you have verified all your applications work with 10.6, then you can go ahead and upgrade to 10.6.6 or later to use the Mac App Store.
Your machine is the minimum MacBook that can install 10.8, but you'll need to upgrade to 10.6.8 first, and make sure all your applications are Universal and have no PowerPC requirements, and make sure you have a minimum of 2 GB of 2 GB of RAM, and 15%+8GB of hard disk space free.
For more on 10.7 compatibility see: https://discussions.apple.com/docs/DOC-2465 and 10.8: https://discussions.apple.com/docs/DOC-3761
Don't ever use torrents, limewire, or other peer2peer software to download operating systems. Their safety can't be guaranteed.