I think that stuff has to come a bit spontanious, maybe your friends don't like the exact same music but there are prolly some things you both like. Try jamming a little, then jam with their friends, and friends of their friends etc. One things leads to another. I been in bands with people I hardly knew personally at first, one day we just started playing and it was good and the chemistry was there so we had a band. Hang out at performances, you know, the little backroom bands, catch rumours of who's looking for what kinda musical direction, get to know people etc.
It also depends on what you play, but you say you're a singer so that's not very hard.
humm well I'm a singer too so basically I have it easy... when I was playing guitar and I screwed up the whole band was lost, now all I can do is forget the words (but that never happens with songs you write yourself- at least in my experience) or sing a bit out of tune and most people don't even notice that
But to be a singer you do need balls, you're in the spotlight almost all the time! So don't be all scared. That also goes for looking for a band. The best way I think is to jam with friends or get to know their friends and others who like the same music. You can reply an ad to of course but personally I'd never do that because you have to go prove yourself to them and it's like applying for a "real" job and all serious and that makes it less fun, but that's just my opinion. I think there are more than enough bands out there that are still looking for a singer or could do with a much better singer than they have now. Just try and ask around. The band I'm in now didn't even have a singer at first, it was pure musically! And they were practicing for a performance and me and this chick were just hanging out around there and we heard the music and checked it out and afterwards we asked why they don't have a singer and they answered that they never thought about one, it wan't nesecarry, but they could use one if someone volenteered. I didn't even know the guys and I just joked I'd do it, we hung out and talked about music and after that night I was the official singer, so you see it can go in strange ways sometimes... I say you just ask around and I'm sure something will pop up!
How do you know if you're good enough? I guess you can ask your friends but don't depend too much on other people. Some will LOVE your voice others will think the neighbours cat does better than you. For advice on how to improve your singing skills there are other threads around here so I won't type everything out for you. But if you thing it's ok and you're happy about yourself then go for it! Someone once said that the perfect artist has the combination of an endless arrogance and a huge minority complex. Be certain of yourself and what you do but don't get a big head. If you feel you're cool then you prolly are, so don't listen too much to what others say.
But don't be too perfectionistic about your bandmembers. Not everyone will want to play as much as you do and at the same times as you do and some will listen to a bit more bluesy music and others to techno but you can still make great music together. So don't worry if you can't immediatly find someone who is as much into the same things as you, different inputs make a band interresting and after all you don't want a bunch of clones either now do you?