like Katrina said, visit QLH
1. Read the stickies
2. Read a bunch of ongoing query threads as well, so you can see what sort of maneuvers are getting panned and a discussion of why
3. Critique. You need 50 posts for #4 anyway, and yes, you can crit even if you haven't written a query letter. You may even offer the "wrong" advice, if so someone will suggest it is against conventional wisdon/agent comments/whatever else, and why....you have a safety net in that, and you know enough to crit what appeals or does not, typos, etc. presumably, or you'd have never finished a book of your own
4. When you hit 50 posts (better if you crit to a couple hundred, but if you can't wait....) sub your query for others to look at
5. strap in and hold on
(It is worth pointing out first-time crite tend to sting; someone just bailed last week over it. critiquing before you sub your own work can help you avoid some of the stinging mistakes, and also help bring home the point you aren't being "singled out" when your glowing work of art is called into thorough question. So hopefull you'll have some idea of the process by the time you get crits of your own work. In either and any case, if you see something you don't like, take a day or two to kick it around before you decide how to deal with it.)