With the game structured and the starting conditions explained, your campaign can get under way. There may be discussion among players as to who will play what role in the party.
There is a school of thought that believes in party balance – but a balance struck by covering the obligatory bases, without exception. The Healer, The Tank, The Spellcaster, The Leader, The Skills Monkey. At least one of these must be present in the party, or so the prevailing wisdom demands. Most prominently this mentality pervades MMO’s and 4th Edition D&D, but its essence is not unknown to v.3.5 or other RPG’s.
If players want to even out their party’s balance, that is their choice. Assuming of course, that players are not forced into a role that does not interest them. A group that has four players who all very much want to play a Fighter or Rogue should not coerce a fifth player into being a healing Cleric or a stat-boosting Wizard because the party “needs” one. The DM can introduce an NPC associate to fill a “necessary” role if it will permit all players to choose a character that they actually want to play.
This NPC can play a valuable role for the DM. Besides being the DM’s subtle proxy within the party, such a character can also help with two critically important structural aspects of the newly minted party: How the characters meet, and why they adventure together as a team.
How do the characters meet ? The first adventure of a new campaign is awkward like a first day of school or a new job. There are plenty of unfamiliar faces, the social order is not known or not established, motivations are unclear, and there’s a measure of defensiveness and caution in all interactions.
Usually the first meeting of all characters comes about through a Mutual Threat or a Mutual Obligation. The Mutual Threat is the peasant in distress, the fight that breaks out in a bar, the goblins that raid the village – something that happens when all the characters are nearby. Putting the individual characters up against a mutual threat demonstrates the characters’ motivations and a sense of how honorable or heroic these strangers are, to the others. NPC’s can play the role of victim or antagonist, as the DM requires.
The Mutual Obligation, while not as binding as the trial-by-fire Mutual Threat, brings the characters together by vetting each through a third-party. Four dissimilar characters, each completely unknown to the others, form a functional party (at least to start out) by agreeing to assist someone. That NPC could be a member of royalty; a Guild to whom all characters share allegiance; a mentor / teacher / trainer; a mutual friend; even a family member or clansman. Characters agree to work together because a reliable source has given them assurances that they new associates are trustworthy.
Why do they adventure together as a team ? Clearly the players want to be in your campaign, and want their characters to function as part of a team (apart from their hidden agenda or future plans). So the players will jump at any chance for their characters to go ahead and agree to keep hanging around and helping these other complete strangers that they totally just met – even overlooking the absurdity of such a fast friendship in a competitive and conflict-driven realm such as your RPG provides.
It is a good idea to spark the Inciting Incident of your campaign in the first adventure. This event, which starts off your campaign’s story and gets the ball rolling, need not be the situation in which the characters meet each other. The characters might first meet when saving a poor wretch from corrupt town guards; then later, they hear a rumour of undead sightings. This leads into your story about a powerful Lich looking to control the kingdom.
Or the Inciting Incident could be what drew the characters together. Each character received a letter from his or her beloved trainer, urging the character to return to their alma mater and take on their mentor’s quest to locate a lost treasure of enormous value.
In any case, the Inciting Incident must happen soon after the characters meet. Otherwise, these strangers would simply exchange thanks for the others’ assistance in the bar fight or express how nice it was to meet them, and go on their merry way. Establish what the objective of your story is, and use that as the reason why the characters have teamed up. The characters must understand that the challenges in the path to achieve your story’s goal are too great to be faced alone.
Bringing the characters together is your first challenge as DM. A mutual threat or a mutual obligation are two ways to make it happen. The second challenge is giving them a reason to stay together on the long term. The Inciting Incident and some idea of their mutual purpose toward an objective (your story) will accomplish this.