28 point buy

It uses standard pointbuy rules from DMG, so each stat starts at 8 and it can't be lowered. You have 28 points to allocate, and the cost associated to each stat is like this: Racial modifiers come after pointbuy.
 * 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 thing about it like this:
 * 1st to 6th ability increase costs 1 point per increase.
 * 7th to 8th ability increase costs 2 point per increase.
 * 9th to 10th ability increase costs 3 point per increase.

Tips
Ability modifiers are more important than ability scores. The key thing to remember about stats is that for almost all purposes, even numbers are the important ones, and odd numbers are only useful to move you closer to an even one. 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 only time odd-numbered abilities are directly useful is for spell points, or as prerequisites for Feats and powers. For example, a Ranger needs wisdom 11 to cast spells, and the Feats Dodge, Two-Weapon Fighting, and Improved Two-Weapon Fighting require a dexterity of 13, 15, or 17, respectively.

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 some Feat preqrequisites.

Character creation isn't the only time to get stat points. They also come at levels 4 and 8, and from racial/class enhancements as well. And there are wearable magic items which boost a stat anywhere from +1 to +4. All of the other ways to increase abilities treat each ability score equally - it is only in point-buy where going from 16->17 is more costly than from 8->9. Therefore, stay away from high attributes at creation if you can, and use 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 hit/damage. But if he had started with 16, then at 8th level he'd have 18 and +4 hit/damage, almost as good... and he'd have 6 points saved at creation, which could be used, for example, to bring wisdom from 8 to 14, for +3 to Will saves and a 15% less chance that a single spell will paralyze him.

Also, be aware that items which give a +1 to a single stat are pretty common, and you can easily find at least one by level 2. 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 +4 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 know what kinds of items / enhancements will be available in the future. Keep in mind though that many useful feats like Combat Expertise, Power Attack and Dodge have odd ability prerequisites. Only your starting ability points and the 2 extra ability points from levels 4 and 8 are taken into consideration during level up. It's a good idea to make sure during character creation that you fulfill the requirements of your favorite feats.