The advice that's been given so far is good advice, but it leaves out one important aspect:
1. Decide what you want your character to do.
So you're playing a wizard. Do you want to shape the battlefield, buff the party, control people's mind, kill foes with a word, or blow stuff up? As a wizard, you're going to have to do more than just your specialty, but you need to pick something to be good at. After you've made your character good at that, then you consider what else he can do.
For instance, let's say you want to blow stuff up in big areas. You'll be casting spells like fireball, flaming sphere, etc, most of which have saves, so you'll want at least a 16 int. You'll also want to think about feats like spell focus, greater spell focus, and empower spell. After you've done that, you can look at your capabilities and figure out that you can use area control spells like solid fog and wall of force and your high int makes your save or die/control spells pretty effective too.
On the other hand, let's say you want to control the battlefield. You'll be casting spells like web, solid fog, sleet storm, wall of force, etc, most of which either have no saves, or work almost as well even if the enemy saves. (Web is one of the latter). You don't need spell focus, and only need int for bonus spells, so you can afford to leave it at 15 if you want. However, the best time to effect the battlefield is before anyone's had a chance to move, so initiative will be a priority for you. Improved Initiative is a good feat for a character like this and Sculpt Spell (Complete Arcane, so you'd need to wait until level 3 to take it in Living Greyhawk) is very good too. If you have a mediocre or poor int, you won't be too good at save or die spells or blasting spells, but a good dex means that you can make ranged touch attacks quite effectively and buffing your party members works just as well as it does for anyone.
Since you mentioned Living Greyhawk, there's a second thing to consider:
2. You won't always play with the same group, so you'll need to have a reasonably flexible character. If your character is set up so that you do nothing but Bull's Strength, and Enlarge Person, you'll be up a creek when you're in a party with a bunch of weapon-finesse using rogues. If you're all about the battlefield control spells and you're in a party full of other controllers without any real damage dealers, you'll be in for a rough time. In a home campaign, you could develop your strategy around specific party members: "I take sonic weapon for the archer with rapid shot and mage armor for the monk." In Living Greyhawk (or any other living campaign), you can't develop your strategy around a specific group member--you have to flexible enough to be effective with whatever group you fall in with.
