Where to start?

I am going to assume that your guild is established. You have done the hard yards, you have a member base, a reputation and you can almost fill a 25man raid currently (or you are running 1 to 2 10man IDs a week). That is the easy part. No really. Getting to this point is easy. You spammed trade chat and got members. They are probably not GREAT but they are learning to work together.

Your next task is to identify what you need in a raid. Generally, raid makeup is something along these lines:
3 tanks
7 healers
15 DPS (5-7 melee, rest ranged)
3-4 bench (RL happens!)

New wisdom on DPS is that you can bring more melee along. TBC it used to be very strictly 5 melee 10 ranged. Since the group thing has been relaxed and a lot of abilites are now raid wide, this has allowed for more melee to step up to the plate (which is good, because on the whole I find melee to be more attentive players, but more on stereotypes later).

So figure out who your "reliable" players are. Reliable means a few things and I will put up a different post on that topic. Figure out where your short fallings are in your raid and RECRUIT HARD. NOW is the time to do it. Spam LFG, Trade and the realm forum on forums.warcraft.com get your name out there and what you want to do.

The most important thing right now is for you get get a routine in. Get 25 people together in your guild and get into Naxx and start. Then do it again tomorrow. Set up a raiding routine so people can plan around it. If you dont have a website, try guildomatic for starters, use the in game calendar, but above all else, get that routine set in stone.

Once in the door a lot else will happen. But for now, get in the door and keep that door open.