You may be able to purge your record, but it WILL cost you, and quite a bit. Expunging your record may cost up to $2000. There is no way around that. The law regarding expunging criminal records varies not only by state, but by county, and maybe even by city. So, your lawyer will have some questions to ask you before you can even find out if you are eligible to expunge your record. I'm inclined to think that you are.

Look, ya fucked up and you have to pay for it. It happens. Don't feel too bad about it. You will have to admit to underage drinking on your job applications in the future (if they require you to list misdemeanors -- some only ask for felonies). Do not neglect to do this. If your employer runs a background check and finds that you were not truthful in your application, you can be charged with perjury, and you certainly won't get the job. Once the community service or fine or whatever happens is over, that's the only thing you really need to be concerned about. But, like I said, you can have it expunged.

Be as kind and courteous to the judge as possible. Show respect at all times, speak only when spoken to, and dress well (business casual, tie is optional) for the court hearing. The more you impress the judge by your otherwise upstanding character, the more likely he will be to consider reducing the fine. The maximum the judge may fine you is $100. I personally would not hire a lawyer, I would simply opt for the public attourney which you are entitled to. The only reason why (aside from lawyer fees) is that the sweet-talk of a good lawyer may help your case, even when you are in clear violation of the law, but I think you, yourself, can present the best character possible.