It's a really bad idea to post your email address - it's an invitation to spam - and I've asked the Hosts to remove it.
The current OS is 'El Capitan' - 10.11 - and is available in the Mac App Store in Applications: it's free. However the Store will not download this if it detects that your computer is not compliant with the requirements, which are
- OS X v10.6.8 or later
- 2GB of memory
- 8GB of available space
and the supported models are:
- iMac (Mid 2007 or newer)
- MacBook (Late 2008 Aluminum, or Early 2009 or newer)
- MacBook Pro (Mid/Late 2007 or newer)
- Xserve (Early 2009)
- MacBook Air (Late 2008 or newer)
- Mac mini (Early 2009 or newer)
- Mac Pro (Early 2008 or El Capitan
If your Mac doesn't comply with the requirements you may be able to upgrade to Mountain Lion (10.8) which is available in the Online Store:
http://www.apple.com/shop/product/D6377Z/A/os-x-mountain-lion (USA - $19.99)
http://www.apple.com/uk/shop/product/D6377ZM/A/os-x-mountain-lion (UK - £14.99)
You will receive a code to enable you to download it from the Mac App Store. Some people consider that Snow Leopard is preferable to Mountain Lion in various ways, but unfortunately it's no longer supported and many programs and websites won't work. The systems following Mountain Lion - Yosemite and Mavericks - are no longer available.