What is my best course of action if my daughter got a DUI, possession of less than an ounce of marijuana and possession of alcohol underage?

Your best course of action is to get her a criminal defense attorney.  This is not a 'go to jail for the rest of your life' case... however, it can have a lasting impact on her life considering her young age.
A defense attorney can get a copy of the offense report and make sure that the stop was a good traffic stop.  Usually, traffic stops are tough to defeat, but you always want to see what the officer puts in the report.  The second component the attorney can review is what are called 'affirmative links.'  It sounds like the items were found in her vehicle, but this does not automatically mean that she is guilty of the offense.  Instead an officer can review the details to see if these items were found in a friend's purse or in an area that your daughter may not have had ready access to.  A third issue for the attorney is field sobriety tests.  These are considered scientific tests, however, because of human error, the result may not be accurate.  An attorney can confirm the validity of these tests.
Finally, assuming that your daughter is guilty, a defense attorney can help negotiate the best deal possible for your daughter.  More and more jurisdictions are offering diversion programs for young offenders.  This is where you daughter earns a dismissal of charges by doing certain things like community service.
Your daughter can show up and just enter a plea... however, a guilty plea can affect her ability to get certain jobs or qualify for scholarships.  She really needs help to make sure that she gets the best deal possible that will minimize the impact on her life.