Don't ever plead guilty unless you absolutely have to. There are murderers who plead not guilty so when in doubt say either not guilty or no contest.
Showing up for court dates(even if it's not mandatory) will most likely reduce your fines/points/whatever, especially speeding tickets. You will have a warrant for your arrest if the ticket says mandatory court appearance and you didn't show up.
Underage drinking ticket is like a mandatory fee to pay during college hahaha. I had one... $230 fine and it's on my record.