At the character creation screen, you will see 30 points free to be put into any stat. You cannot take the stats below their starting level; nor can you just put all 30 points in one stat. The first 10 points you put into an attribute will give an increase of 10 points. If 10 more points are added, the stat will increase by 5. Yet another 10 points will increase that stat by only 3 (with 1 point left over). It is best to spread out your points, unless there's a particular stat which you need especially badly.

It doesn't really matter what race you are, as far as stats are concerned. No available races have any stats so high that the cap will provide a specific problem. The only stat which might be a problem in that regard is Strength, which, regardless of race, will be close to the soft cap when fully buffed at level 50.

The first thing to consider is that Reavers do not gain Constitution as they level, and low hit points (and longevity in general) become the Reaver's biggest problem at higher levels. Therefore, at least 10 points of your 30 should go into Constitution.

Strength doesn't require any points, as it raises every level. By the time you reach level 50, you will probably have close to or above the 300 mark. While 300 is no longer a hard cap, it makes little sense to put too many points in it when you have other stats that are not close to the 300 mark.

Dexterity is an important stat, especially for Flex Reavers; not only does it increase all of your defensive abilities, it will increase damage when using flexible or thrusting weapons.

10 points each in Constitution and Dexterity is the easy part. Choosing between Quickness and Piety is not so simple.

First, though many believe otherwise, Quickness does indeed help. While it's true that more Quickness means that your style damage will decrease per hit, a Reaver doesn't rely on styles for most damage. For a more thorough explanation, please read this page. The effects of Quickness are fairly small; 10 points will shave off only around a tenth of a second from your attack speed. That tenth of a second adds up after a while, however. Also, Quickness does not seem to decrease the damage done by style proc effects, such as Cobra and Leviathan.

Piety, meanwhile, helps with your spells. It will not only decrease variance, but increase your Power pool, allowing you to cast more spells or run chants longer. The effect is as small as that of Quickness.

In the end, the choice between Piety and Quickness probably will not matter much. A much more visible difference, in fact, would come from adding another 5 points to Constitution. The choice is a personal one, between melee offense (Quickness), defense (Constitution), and magic (Piety). It'll have little effect on your character when you reach the end-game, so just go with what feels best.