What if you're an under age drinker in Miami County, OH (age 18), what are possible charges when appearing in court? Should you have a lawyer?

Whenever possible its best to have a lawyer there with you because there are things you may be facing that go beyond most common rumors. In some states if your caught drinking underage then they can revoke your driving privliges until your of legal age. And it doesn't matter if you were driving at the time or not. So yes get a lawyer to go in there with you.

I have a close family member (also in OH) that ran into this. They did not use an attorney. They pleaded guilty and got put on probation. It ended up being a misdemeanor.

Check out this related thread:
https://www.askmehelpdesk.com/other-law/underage-alcohol-consumption-156376.html

... what are possible charges when appearing in court? ...

You were served with paperwork? It should say the possible charges.