i have os x leopard can i upgrade to yosemite and if so how?

See this page -> https://www.apple.com/osx/how-to-upgrade/

Take a look at these support articles and download the combo update in case you face any difficulties:

https://www.apple.com/osx/how-to-upgrade/

OS X Yosemite 10.10.3 Combo Update

Check that your computer is compatible with Mountain Lion/Mavericks/Yosemite.

To check the model number hold down the option/alt key, go to the Apple menu and select System Information.

• MacBook (Late 2008 Aluminum, or Early 2009 or newer) model number 5,1 or higher
• MacBook Pro (Mid/Late 2007 or newer) model number 3,1 or higher

Your Mac needs:

• OS X v10.6.8 or OS X Lion already installed
• 2 GB or more of memory (More is better - 4 GB minimum seems to be the consensus)
• 8 GB or more of available space

Check to make sure your applications are compatible. PowerPC applications are no longer supported after 10.6.

Application Compatibility

Applications Compatibility (2)