Decide what instrument you will play in the band. Depending on what that may be, you must find the other members to complete the group. If you are a guitarist, which is the most popular position, you must find a drummer, bassist and vocalist who share a similar vision.
Find an adequate drummer. This is essential because metal is rooted in fast and heavy drumming, and drums are the most important instrument in metal. Although metal involves loud guitar, your band is only ever as good as the drummer. You must be very picky when choosing a drummer as many specialize in one certain style and you must pick someone who compliments the style you wish to play. Double bass is a must in metal.
Look for a bassist who can play articulately and that has equipment loud enough to play metal. One thousand watts is a good starting point, and either a four- or five-string bass will suffice. Although it is not imperative that your bass player is a virtuoso, he must be able to lock in with the drums precisely.
Find a vocalist that is comfortable with your band's style and that has a good stage presence. This can be tricky because there are many styles of singing, from a Judas Priest-style falsetto to a Slayer-style scream. Metal is also about entertainment and you do not want your vocalist just standing there when your band plays. You want him to have control of the crowd during your performances.
Add a second guitarist to the mix. Many metal bands use two guitarists to increase the fullness of their sound. This addition also allows for harmony guitar work as heard in many popular metal bands like Metallica and the aforementioned Judas Priest.