I would advertise locally at places the kind of person you are looking for would frequent. (Although I wouldn't put an age limit on it you might find someone a little older than that that would be great for the position and have excellent experience).
To get started, I would advertise for people who already play the instruments you want in the band/a lead singer. Remember, many people who play an instrument also sing as well. I wouldn't look for a manager right away because most of them want to be paid and you probably won't have the money for that right away. Be ready as the person who started the band to work hard to get it gigs right away.