Starting a raid is very similar to starting a party. However, now that you and your friends are level 50, you have access to a new option when forming a party.
Clicking on form a raid will change your party window into a raid window as seen in the following picture:
Ideally, the recommended raid composition is two tanks (Main Tank and Off Tank), about 4 DPS (range dps or melee dps), two support classes, and two healers.
The Main Tank role can be played by nearly any of the tanking classes: Knight, Warrior, Beastmaster, and Monk. While some classes are slightly better than others in terms of tanking, it's all a matter of preference and any of them are more than capable of filling the role of main tank. Their job is simple, keep threat on the boss you're fighting, and try to keep the boss facing away from the raid group at all times.
The Off Tank role, like the main tank, can be played by the same four tanking classes, however the difference in ability to offtank is much larger than what's required to main tank. Warriors and Knights are much better at off tanking than Monks and Beastmasters, however, this isn't to say Monks and BMs are incapable of the job. It all comes down to personal preference in the end, and skill will greatly outweigh what your class is capable of doing.
A majority of the Off Tank's job is to control the adds that spawn from the boss, and serve as a back up tank in case the Main Tank happens to die for whatever reason. There will also be some fights that require the MT and OT to swap threat.
DPS is fairly simple: You are the main damage dealers. You hit fast, you hit hard, but you're also super squishy. Regardless of whether you are a ranged DPS (Ranger, Wizard, or Sorcerer), or melee DPS (Rogue, Assassin, Crescentia, and in some situations, some of the tanking classes).
A good DPS will be able to effectively switch between dealing damage to the boss, and clearing out adds fast to protect their raid group depending on the situation and which takes priority over the other in the fight.
The Support role is primarily for Sorcerers and Soul Makers. As the name implies, your role is to provide the group with extra healing or extra damage depending on what's required of you in the situation. It's possible to place priests in this spot as well, but remember that unless the player is specifically a battle priest, you'll be losing out on some DPS if your party is lacking.
Ideally, your role as support is to make sure you, the DPS, and the healers stay alive.
The Healing role is primarily for Priests, though there are situations where a Full Support Sorcerer can do decent, if not just as well as a priest in certain situations. Most of the time, the healers are going to focus on keeping the tanks topped off in health, while providing support for the rest of the group where needed.