I'm a novice at best, so take it for what it's worth - but I either start with a lyric I really like, and try to come up with a melody that matches the tone I want to surround the lyric, or I start with a kick a$$ riff / melody - try to see what that makes me feel like and build lyrics around that, expressing the same tone. Both are centered around the 'feeling' I want the song to have.