One of the main key's here-is that you are already singing in a band-which means that you've gotten over the "fear" of singing in front of others.
The difference betwen a singer, and someone who sings-is that the singer doesn't care if someone else doesn't like his/her voice!!
That's just someone else's opinion, to them.
I think that just about everybody can sing-alot of it is that people tend to "hold back" for fear that someone else will make fun of them.
(This was my case) I recorded a demo for some friends-and I bounced a copy for myself, and laid vocals to it.
I gave it to them as a joke-and they wanted to kick out their singer and get me to sing in their band!! I've had several bands asked my to sing for them.
All it really took was getting over the fear of being mocked-and being willing to "commit to the note"; that is, to sing with conviction and emotion, and not sing things "weakly"/halfheartedly, in a "shaky voice"-like people who are afraid to speak in public.
Go to you're local college, and I would also suggest taking a course or two on Public speaking-if you're going to sing in front of people. you need to be able to speak to them as well!!
I know that for me, Singing is like acting. When I go on stage-I take on a persona so to speak, and that persona only lives there; that persona is a rockstar, and doesn't really care what anyone else thinks of the performance.
I know this might sound strange to some people-but it's just "playing a role".
I agree, that singing other styles of music will help.
I play in a Neo-Classical/Progressive metal band where I am the Drummer AND The lead singer (actually the only singer-no background vocals), BUT, for my own amusement/ pleasure/whatever you want to call it-I also sing country songs and pop songs, I especially like The Goo-Goo Dolls.
and practicing singing the Goo gooDolls' songs , ahave helped my work out the "breaking points" in my range when I shift from my "Chest voice" to my "head voice", and from my "headvoice" to my "falsetto"...
Chest Voice would be like Glenn Danzig sings; where you're using your chest for all that resonance, and lung power.
You need to learn to contro lyour Diaphragm, and how to stretch your vocal chords, instead of "pushing" them.
DO NOT, do any "death metal" or pantera-ish Screaming!!
If you plan on dooing that-then forget any training at all-because that style has been known to literally cause the vocal chords to TEAR! I know a singer who did that for a long time, and right in the middle of their set, started spitting blood! you guessed it-Torn Vocal chords.
Now this singer has Half the range as before!!
So, if you're gonna do the Death metal thing-invest in a harmonizer, and learn how to work your own range-and stay within it.
At low notes the vocal chords are more "slack" and thicker than at high notes-and the quick "push" from do-ing that "demon growl/howl" vocal style can cause major damage.
Tim