Snow Leopard 10.6 system requirements:
Mac computer with an Intel processor
1GB RAM (2GB or more preferred)
5GB available disc space
DVD drive for installation.
You can obtain the retail DVD from;
If not in the us, replace the country code after Apple.com with the local one; i.e. /uk/, /de/, /fr/ etc.
When you've installed that, update to 10.6.8 using the combo installer; http://support.apple.com/kb/DL1399 That will give you access to the Mac App Store.
Be warned that printer drivers, and scanner drivers in particular will almost certainly need updating for SL. That could be a problem if the printer or scanner is more than a couple of years old. Some printer updates are available from Apple Support Downloads.
Many third party applications (and some Apple ones) may need updates for SL (Adobe particularly), so check with the vendors for compatability before you upgrade.