Decide on the genre of music and songs that you feel most comfortable singing. A keen harmonic and melodic sense is more important in a Top 40 band, less so for the local punk rock combo. Either way, try to resolve these issues now, since your persona will more or less define whatever band you join for the long haul.
Learn as many songs as you can. Not all situations require original material, but even if they do, there's no substitute for a working knowledge of musical history at your fingertips. (It's worth recalling that early Beatles sets included such unlikely one-offs as "Besame Mucho.")
Enter as many talent competitions, open-mic nights and other public forums to hone your style--at this point, the confidence and the exposure outweighs whatever you're getting paid. In any local scene, the better opportunities go to those willing to make the rounds.
Create a demo packet once you've carved out a local niche for yourself. Choose three well-recorded songs that showcase your style and vocal range, plus a photo and brief biography summarizing whatever experience you can offer.
Compile a list of bookers, managers, promoters and venue owners who are plugged into the usual opinion and rumor mills, much of which you'll learn to discount while awaiting the tidbits that matter.
Browse bulletin boards at colleges, music stores and record shops--as well as Internet sites like Craigslist--to learn who's hunting for a singer. Make up business cards you can pass out to musicians or other influential people who might be able to help you.
Post a profile and sound clips on MySpace, or other music-related pages, to maximize the Internet's worldwide reach. Choose each outlet carefully to ensure a good fit for your talents, and keep the actual number of them manageable, too. Regular updates are crucial in attracting and retaining new fans.
Choose a band that you want to join. Obtain a demo CD of the material, and make a point of seeing your potential future colleagues performing live, too. The last thing that anybody needs is a mismatch of skills and abilities that doesn't do either side justice.
Memorizing lyrics is just half the battle. Listen closely to the demo arrangements to see how your vocal style interacts with the other instruments, since it's likely that your input won't just stop with the lyrics.
Don't skimp on rest before the big day, and make sure to warm up before having to sing. It's natural to feel a little nervous, but you don't want that mood overtaking you, so you end up beating yourself.