I am yet to make a mod that alters the tool tip, but the best advice I have for you is to read the code of some addons that do it, and then fiddle and poke them.
Check out the documentation at
http://wowprogramming.com and
http://www.wowwiki.com/World_of_Warcraft_API
Some addons that modify the tooltip:
Engravings
TipTac
ManaPerc
Also, check out LibExtraTip. Via
norganna SVN, it 'Assists in the management of additional tooltip information.'
Also, for real-time help (at IRC speeds), visit the WoWInterface community at irc.freenode.net#WoWUIDev.
And do you have
The book? Recommended.
If you still cannot understand how to do tooltips, try something simpler and work your way up to that.