Good advice particularly hubberjub, katastrophe & vroomery. The techniques katastrophe describes were what I learned a lot from working with a co-writer who was an ace with constructing harmonies. I find a piano or keyboard is a much better aid than a guitar for identifying intervals.

But no matter how your voice sounds, pitch-wise, tonally...it's about breath control & techniques.

Here's a link to a book/CD course that's very good. I got it for my daughter years ago on recommendation from her HS Choir teacher. Even though she's got a natural ear (pitch, harmony) developed from being immersed in music since she was about 4 yrs. old, singing in school & church choirs, etc., she wanted proper technique. And while she got that from her teacher at school, she wanted something she could practice with at her own pace, at home.

Seth Riggs Book & CD Singing Course