28 point buy

DDO uses the standard point buy rules from pen-and-paper D&D, so each ability score starts at 8 and can't be lowered. You have 28 points to allocate, and the cost associated with each score is like this: Racial modifiers come after the point-buying step.
 * 8:0
 * 9:1
 * 10:2
 * 11:3
 * 12:4
 * 13:5
 * 14:6
 * 15:8
 * 16:10
 * 17:13
 * 18:16

Or you could think about it like this:
 * 1st to 6th ability increases cost 1 point per increase.
 * 7th to 8th ability increases cost 2 point per increase.
 * 9th to 10th ability increases cost 3 point per increase.

Tips
Ability modifiers are more important than ability scores. The key thing to remember about abilities is that for almost all purposes, even numbers are more important than odd ones. That's because ability scores mainly take effect through Ability modifiers. Ability mods start at -2 for a 6 score, and increase by 1 for every 2 points above 6. A Fighter who takes a 16 Strength instead of 15 will notice more hits and more damage, but going from 16 to 17 will have absolutely no combat effect.

The biggest ways that odd-numbered abilities are directly useful are in spellcasting and as prerequisites for Feats. Casting classes need an ability score of at least 10 + X to cast one of their level X spells. As for feats, several have odd prerequisites. For example, Dodge, Two-Weapon Fighting, and Improved Two-Weapon Fighting require a Dexterity of 13, 15, and 17, respectively. One other case where odd abilities matter immediately is Strength, which improves all characters' carrying capacity for every point.

But in general, only raise an ability score to an odd number if you expect to be able to raise it again later on (see below for ways to get it raised). Aim to have all abilities land on even numbers when your character is maxed-out, unless you were exactly meeting feat prerequisites.

Character creation isn't the only time to raise ability scores. They also come at levels 4, 8, 12, 16, and 20, and from racial/class enhancements as well. And there are wearable magic items which boost a stat anywhere from +1 to +6 and tomes that give single-stat bonuses of +1 to +4 (though +3 tomes are rare and +4s are almost unheard-of). All these additional ways of increasing abilities treat all scores equally - it is only during creation where going from 16->17 is more costly than from 8->9. Therefore, consider staying away from extremely high attributes at creation and using other means to get abilities above 16.


 * Example: a fighter who starts with an 18 strength can increase it to 20 at level 8 (before enhancements), for +5 to attacks and damage. But if he had started with 16, then at 8th level he'd have 18 and +4 attack/damage, which is almost as good and costs 6 fewer build points at creation, which, for example, could be spent instead to bring wisdom from 8 to 14 for a net +3 to Will saves and a 15% less chance that a single spell will paralyze him.

Important! When a feat requires a minimum ability score, enhancements and item bonuses don't count toward it. You only get to count your score at creation (including racial adjustments) plus tomes and level-up bonuses. Make sure you take this into account and either raise the ability high enough during creation or get it high enough that you can raise it the rest of the way with methods that actually help meet the minimum.

Finally, be aware that items which give a +1 to an ability are pretty common and easily found by level 2, and that you will be limited to +3 or +5 items at some levels. Even if you are using some stats as "point dumps", it might help to leave them at 9 instead of 8, to take advantage of those fairly cheap items. On the other hand, at high levels you can occasionally find items giving +6 to a stat, making that one odd-numbered point no longer useful. Therefore, it is difficult to guess the optimal value at build time, since you cannot always know what kinds of items and enhancements will become available in future updates.