The first practical thing to consider is how combat-oriented you want to be as a player. If you can't wait to hurl yourself at massed formations of goblins and other monsters, chances are that playing a scholar isn't right for you. Bear in mind that everyone must NPC a bit, and that more than likely you will portray monsters and villains during this time. For some people, this is enough combat to keep them happy for an event, while others prefer to fight as their PC. If you have a clear idea of how much combat you'd like to experience, then you can begin to narrow down the character choices.
There are generally three types of characters: low-combat, semi-combat, and full-combat.
Low-combat characters are mages, priests, healers, alchemists, and other similar professions. Some low-combat characters can still have a very active role in a combat situation, as they wield the power of spells instead of the power of a blade. Since combat is an essential part of KR, be advised that a low-combat PC may be boring unless you have some very specific ideas that will keep you occupied throughout the weekend.
The most common semi-combat characters are rogues (thieves, assassins, spies, etc.) and smiths. Most of the other professions can be stretched one way or another into a semi-combat role, by jockeying your skills and character concept. If you want to play a priest or mage, your choice of deity or spheres will affect what you can do, and may take you from low-combat to semi-combat.
Full-combat characters are any of your warrior-types. Their skill lists are comprised almost entirely of different combat skills, and they can accumulate more body points than the other professions. Despite their focused nature, full-combat characters still offer plenty of opportunity for role-playing.
Once you have a profession down, think about a race. Some professions are restricted to some races, so that can be a factor. Also, make-up and garb is a consideration. Humans are always easy to pull off, and elves are not difficult (just the ears). However, races such as drow and goblinoids require more extensive makeup, or even prosthetics.
Lastly, consider skills. What skills you take can greatly determine how you play your character. An often overlooked aspect of skills are trades. For a simple 4 Build, you can take a trade that helps define your character and makes him or her useful In-Game as well.
When you're ready to actually begin writing your character out, check out the Forms section of the Downloads area of the website. Here, you will find the Character Builder worksheet, along with the Character Background worksheet, to make your life easier.
