Create a Guild Raiding Calendar
Consider creating a calendar of raid times that takes advantage of when your guild has the most members available. Let guild members know well in advance when a raid is scheduled to occur so that they can plan accordingly. You can create a sign-up sheet and ask players to sign up in advance. This way, you will know who intends to show up and can organize the raid more effectively. There are also free web-based calendar programs you can use on your personal guild website as well. At the very least, having a regular raid schedule can allow guild members to plan their attendance accordingly.

Raid Scheduling
It's often a good idea to schedule raids around the times that the raid dungeons reset. You can see when each dungeon resets here. You should also try to pick times that align with the play schedule of the majority of the guild. Be flexible with your start and end times, and be prepared to substitute people in and out as the raid progresses, since people may need to arrive late or need to leave early from the raid.

Raids Spanning Multiple Days
Initially, when a guild makes their first attempt at progressing through a raid dungeon, it can often take several hours to advance from boss to boss. As the guild begins to learn each encounter and perfect their raiding technique, players will find that the time it takes to complete the raid dungeon is significantly reduced. Many guilds work toward this goal so that they can cut down on the time investment of a particular raid dungeon and also schedule in additional events and raids. Until your raid group reaches that point, it's good to remember that most raids can be completed in increments. A boss kill is usually a logical stopping point for a raid group, before continuing on another day.

Still, it's important to try and schedule raids with plenty of time to allow for completion. Otherwise the dungeon will reset on its scheduled day and you'll have to start from the beginning. Consult the Raid Calendar when planning your raids to help make sure that you can complete as much as possible before the time of reset. Most importantly, be patient with yourself and your guild if you find that it takes some time to learn a particular dungeon, especially if you're at the point where you can't complete the dungeon before it is reset.

Selecting the Raid Dungeon
Initially, you want to focus on raiding dungeons that benefit and better equip the majority of players. Once you have built up your guild's equipment and gained experience working as a group, you can work to take on more challenging dungeons.

Upper Blackrock Spire (UBRS) and Zul'Gurub (ZG) are good places to begin. Upper Blackrock Spire allows 10 players to be in the instance at one point in time, while Zul'Gurub allows for 20 players. From there many guilds head to Molten Core or Onyxia. Both of these encounters allow for 40 players to be in the instance at one time. Ruins of Ahn'Qiraj (AQ20) is a good dungeon for raids that have mastered UBRS, ZG, and Molten Core. Once those dungeons have been mastered, players often challenge themselves with Blackwing Lair (BWL) and Temple of Ahn'Qiraj (AQ40). Last on the progression is Naxxramas, which currently, is the most challenging raid dungeon in the game. Blackwing Lair, Temple of Ahn'Qiraj, and Naxxramas also all allow for 40 players to be in the instance at one time. If you're looking for non-instanced experiences, there are multiple outdoor raid encounters located in many points throughout Azeroth. These can prove to be fun, and challenging. Keep in mind, since the outdoor raid bosses are not instanced, you may end up competing with other players for the victory. You can follow the link below for more information on the outdoor raid encounters.