Post a couple of things to the WotC forums. Maybe a prestige class or monster or something. Make sure you have a handle on style and format. And make sure you have a nice thick skin for criticism.
Check out some publisher forums to see what people are looking for. Don't discount WotC. They're always looking for cool things, just like everybody else.
Read the publisher submission guidelines. Always. If the publisher is so small they don't have guidelines, assume they use WotC's.
Make sure everything is spelled correctly and has proper grammar. Always. Proper spelling and grammar will be dependent on what country you pitch your ideas to, so pay attention to that.
Have an idea of what the company publishes before you pitch an idea. If you don't see any adventures published by a company, odds are strong they aren't interested in them. My tiny dip into the world of publishing was a little discouraging that way. I had a setting and three guys approached me independently, each one saying "I have this great idea you should publish! OK, it doesn't really relate to your setting, but...."
Prepare to get rejected. Prepare to get rejected often. Prepare to submit several re-writes of the same article only to have it rejected. Don't worry. This happens to everyone.
Know what you're allowed to write about. If you're doing d20 fantasy, use the SRD. Suggesting to a d20 publisher that they publish your big mind-flayer adventure will cause them to print out your email just so they can burn it. A surprising number of publishers don't want the hassle of using other companies' OGC, so even if you tell them how wonderful and legal your manuscript is, they won't care.
Don't be paranoid about people stealing your work or making thousands of dollars off of your idea. The RPG industry is very small and many full-time writers work for less than a McDonald's burger flipper. In the words of Han Solo, "Don't get cocky, kid!"
Remember, every great success story started with a few failures. Writing is a wonderful hobby and you may as well get paid for doing something you think is fun. And trust me, if you have good spelling, proper grammar, and turn your work in on time, you'll be ahead of most people!