I have to say before I begin that I haven't had the chance to play a DnD game(computer or tabletop) in over eight years and have forgotten most of the major mechanics. That said, I killed the Intellect Devourer over two and a half hours ago, and my intelligence is still down by one. How do I get it back up again?

My stats as of a level 4 Fighter are: 15 strength, 14 dexterity, 14 constitution, (normally) 12 intelligence, 12 wisdom and 12 charisma. Like I said, its my first time playing DnD again in a long time and I'm still getting back into it again.

Ability drain is cured by temple services or scroll/spell of restoration. You may also try resting for none 'permanent' losses.

Visit Aribeth (or Nurik if you're already in chapter 2) and ask for healing. As a Fighter, you will never be able to heal this on your own. The Intellect Devourer is an annoying fight. I did the exact same thing on my first NWN character; Fighter with 14 to all physical stats and 12 to all mental stats. It really does feel graceful during character creation, but it's quite clunky in practice. Just keep focusing on your strength score to get your attack bonus and damage up.

Drinking a Lesser restoration potion is a way that any class (including a fighter) can cast a spell on themselves to counter ability, AC, AB, SR & saves drains... just like Hickory already suggested.

When you begin to encounter shadow creatures regularly (not the summons, they don't use drain attacks), a stack of restorative potions of any power in inventory is a wise preparation for any player to observe.

Sorry, I meant on your own power.

You are correct, of course: generally speaking there's a magic item for everything. I'm a particular fan of multi-classing my fighters with rogue to get access to the "use magic device" skill, then using it to cast spells from scrolls and wands. However, you need to be mindful that it's not a class feature and it's something that's draining resources every time you use it.

Talk to Aribeth, in the Hall of Justice. Continue (>#1), and select dialogue option >#3: "I am injured. Can you heal my wounds?" Like magic; the ‘Intelligence decreased’ tag goes away.

Like chipster said, potion of lesser restoration- I believe this potion actually has a scripting issue that makes it stronger than it is supposed to be under 3e rules, because according to the rules it should not be able to remove curses. Curse effects are supernatural (which is the type of malus the OP was suffering under if it didn't go away after resting)... rather than magical. But when Lesser Restoration removes the various penalties linked to a curse, it removes the curse, whoops.

The spell does not remove poison or disease. But the potion of antidote, which says it provides Neutralize Poison, actually removes disease, too.

Are you sure it's a bug and the developers didn't simply decide to bend 3rd edition rules some to make the game more functional? I'm pretty sure that's done in some way or another in all adaptations.