Legalize marijuana. It would save billions of dollars on a pointless drug war we can't win. It would free up prisons, bring in tax revenue, and create new jobs. Not to mention it's a relatively harmless drug with medicinal benefits.
End war in Iraq, national health care, legalize marijuana, legalize gay marriage in all states, and help more kids get to better colleges by making the tuition go down with how many hours they volunteer. (The last one is still just a thought process, but it seems like a good idea )
The first thing I would do is make congress take a pay cut, then I would work on legalizing medical marijuana, cut taxes and BS govt spending.
There so many things you can do as the leader of our country, but my luck I would get bumped off after my first week of office.