Focus on expanding your singing range. Good metal vocalists have wide ranges. The best way to start is by finding the lowest and highest notes you can sing comfortably and work up and down from there. Sing scales. You can find recorded scales at a local music shop. Sing along with the recorded scales until you work your way up.
Learn good vocal practices from the start. This includes how to breathe properly and how to use vocal techniques such as vibrato to add color to your voice. A good vocal coach can teach you these things, or if you'd rather do it yourself, you can get a recorded vocal course put together by some of the best metal singers and teachers in the business (see Resources). An at-home study course can be just as effective as going to a live vocal instructor.
Record yourself singing scales and listen back. It's hard to hear flaws in your singing when you're doing it, but listening to a recording helps you pinpoint where you need work.
Sing live, with a band behind you at full blast. This is one of the key situations a metal singer must be comfortable in. Having the power to perform vocally with a band at full tilt is essential--club gigs are a battleground. Get used to less than ideal conditions regarding stage volume and monitoring.
Work on your high-pitched metal screams, but don't forget to practice melodic singing too. A good metal vocalist should be able to do more than scream. Try singing a variety of styles, including opera and soft rock ballads. Being able to control your voice and make it perform the way you want it to perform is a skill the best metal singers have.