General Strategies
The Neverwinter Nights manual is full of good information. It is highly recommended that you read it. The spells, feats, and skills, as well as character races and classes, are described in detail, and it's a good idea to familiarize yourself with that information.
Planning for a Strong Character
One of the major reasons you should familiarize yourself with the information in the manual is that a good character is planned from the beginning. The 3rd Edition Dungeons & Dragons rules remove most restrictions on the classes you can choose, which gives you a great deal of freedom to make a bad character.
For instance, a half-orc or dwarf sorcerer is going to be crippled from the beginning because both races have an inherent charisma penalty, and a sorcerer's primary statistic is charisma. When creating a character, be sure to choose a race that will bolster the strengths of your chosen class. An elf monk, for example, will have a great dexterity bonus, but at the cost of some hit points.
Multiclassing is also a tricky issue. It may be tempting to take a rogue level to get some extra points in the open lock skill or to take a fighter level to get all of the weapon and armor proficiencies for free. But this may severely hinder your experience gain if you haven't planned ahead. Be sure you plan your character's progress from the beginning if you want to maximize his or her potential.
If you feel you've made a poor choice and you're already well into the game, don't worry. There are plenty of magic items to be found that will offset your weaknesses and enhance your strengths.
Balance vs. Focus
As you acquire magic items, you'll have to make some decisions: Should you try to bolster all of your statistics to decent levels, or should you focus on bolstering one or two statistics to superhuman levels? The answer, usually, is the latter.
A fighter with a high strength is exponentially better than a fighter with a good strength and a good charisma. A spellcaster with a high primary statistic will learn more spells, memorize more spells, and will get to cast higher-level spells. Multiclass characters may want to try to keep their stats a bit more balanced, but other characters will want to try to drive those primary stats through the roof.