I've always believed that the best "earn while you learn" gig is being a street musician. You can polish your singing and playing while people who want to stay and listen will, those who don't will move on. You're not accountable to a bar owner who, is ultimately in the liquor business, not the music business. You can also see which of your songs are being well-received and which ones aren't, all while picking up a little extra money. No breathing second-hand smoke or being up until all hours.