start at a score of 8
1 point for each point up to 14
2 points for 15 and 16
3 points for 17 and 18
so 16 points for an 18.
As a human paly who has access to complete divine and complete warrior (not mentioned, so I'll just assume ) I would go for an 16 charisma. 22 point buy points left for the rest of the stats.. 10 str (2 points), 12 dex (4 points), 14 con (6 points), 12 int (4 points), wis 14 (6 points) then as many level points and as much money on items you can to boost charisma. The extra turn attempts after level 4 will be nice and the boost to all of your saves is incredible.
The only big problem is the lack of strength. Although with some decent armor and a shield you will have a good AC.
But then, being a human, one could easily take two points off of int and two points off of dex and get a 14 str which is pretty sufficient.
All of this depends on what sort of paly you want to play though