The first step in Upgrading... is to Snow Leopard = OS X 10.6.x
It is Not available as a download... It is a Paid Upgrade.
Do this first...
Check that your Mac meets the System Requirements for Snow Leopard...
Snow Leopard Tech Specs
If so... Purchase a Snow Leopard Install Disc...
After the Successful Install, run Software Update to get the latest updates for Snow Leopard.
Be sure to make a Backup of your Current System Before Upgrading...
Next... If your Computer meets the System Requirements...
With Regard to ML... Check here for compatibility of 3rd party Software you may be using...
You should also consider more RAM... Get the Maximum you can for your Mac...
It is important to get the Correct and Matching RAM
See Here > OWC RAM > http://www.macsales.com
Yes, 10.6 is the next upgrade. To get it you must purchase the Snbow Leoard install DVD from Apple.
There is two more upgradesd after that Lion and Mountain Lion. In order to know what your Mac can run you would have to look up the requirements for them and compare to your Mac or provide the details about your Mac to us so we can look it up for you.
You can use this Link to check which RAM is suitable for your Mac...
It is Important to get the Correct and Matching RAM
For best performance, fill all memory slots, installing an equal memory module in each slot.
That site also has videos on how to Install RAM should you need it...
Here are Apples Instructions... Installing RAM
oh, and how do you make a backup copy of your system b4 upgrading?
Before attempting any Major Upgrade...
get yourself an External Hard Drive... and create a Bootable Clone Backup of your current Hard Drive...
By far the easiest way to make such a Backup, is to use something like
or CCC http://www.bombich.com/
That way, should anything untoward happen during the Upgrade,
you will NOT LOSE ANYTHING.
(Get an EHD that is at least equal to your current Drive...)