In order to proceed you have to upgrade to Snow Leopard (10.6.8) first: you can purchase this from the online Apple Store. Note that you must have an Intel Mac in order to install this (and you should check the other requirements, listed in the Technical Specifications on the Store page, before purchasing). As you have Leopard it's possible your Mac is a PPC one: if this is the case you cannot upgrade any further.
Once you have upgraded to Snow Leopard you can obtain Mavericks for free from the App Store in your Applications folder. The requirements for Mavericks 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 newer)
It is available from the Mac App Store (in Applications).
If you are unable to install Mavericks (it won't let you if your Mac isn't compliant with the requirements) you may be able to install Lion.
The requirements for Lion are:
- Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
- 2GB of memory
- OS X v10.6.6 or later (v10.6.8 recommended)
- 7GB of available space
Lion is available in the Online Apple Store ($19.99).
You should be aware that PPC programs (such as AppleWorks) will not run on Lion or above; and some other applications may not be compatible - there is a useful compatibility checklist at http://roaringapps.com/apps:table