There may be editors out there just dying to give you some work, but unless you specifically and articulately ask for that work, they're going to give the work to another writer who they already know exists. The query letter is our chance to both announce and prove ourselves. When starting out, query letters often seem like time-consuming missives sent off into the ether, but you really do need to query if you want paying gigs. The more you query, the more writing credentials you earn and with more writing credentials, the greater success you'll achieve with your query letters.
Moral of the story: just send out some query letters already!
Even though I now have a much higher rate of success with my query letters then when I started out three years ago, the truth is I don't query as much as I should. I find it easy to get overwhelmed by the process, even though I've done it many times.
Here's the basic process of how to write a query letter:
1) Research the publication you'd like to query.
2) Determine if the publication's a good fit for your writing. Be honest about this. Trying to get a square peg into a round hole really is a waste of time.
3) Come up with a brilliant article idea for the publication. Make sure this idea is fairly original and truly a fit for the publication. (You don't want to come off like Hugh Grant in Notting Hill, interviewing Julia Roberts for Horse and Hound.) You'll also need to check the archives to make sure the magazine hasn't recently published something similar.
4) Carefully outline your article idea. This means you need to use specifics - who you plan to interview, how you plan to organize the article, etc. Sell yourself as the best person to do the job by listing your credentials and highlighting anything else that might make you an especially good choice for writing this particular article.
5) Submit - making sure you've addressed everything to the appropriate editor, have a completely error-free letter/email that make it sound like you actually know what you're talking about.
There are many, many how-to books devoted to the subject of query letters. My favorite remains Michael Perry's Handbook to Freelance Writing.
On average, a good query letter should take you at least an hour to complete. It's time consuming, which is why I like to procrastinate with them.