I would quickly note down what exactly would be needed to cure some of our most troubling diseases (say, AIDS, heart disease, breast cancer and strokes.) Then, I would quickly as possible note down what it would take to give my ghost friend Marcus a body again so he could live a normal life. 
And I would want more time, because I’d also like to find the perfect girl for me and where she lives, but Marcus and the diseases are more important.
What is the nature of God?
What is the nature of the Universe? How will it end?
What is the future of my life?
Good lottery numbers?
What, in the blue fuck, do women want?
What (thank you Ron White) do they all look nekkid? 
I would ascend.
I obliterate the universe as my omniscience violates the Uncertainty Principle in all of spacetime, collapses all the quantum waveforms in the universe and destroys all matter in the past, present and future simultaneously.
Assuming that my control is good enough to avoid that little problem, I think the first thing I’d like to know is if there are aliens or not, and where. Are there other universes ? Winning lottery numbers. And, is there any avoidable danger to my life and health that I can do something about ?
Most stuff close to me I don’t want to know too much about, as it would spoil the surprise.
Are you sure you just wouldn’t end up laughing at frogs for the rest of your life?
I would use the knowledge to ascend to omnipotent godhood. I think I’ll take the trickster god role. That sounds like fun. Fart noises from all politicians for eternity. 
I’d be dull and pick each of the lotto numbers that score over £20m for the next five years, that should sort me out nicely.
Some have been mentioned but I think they’re worth restating: cures for the biggest diseases (cancer, AIDS), where the aliens are and what they’re like (I don’t need omniscience to know that it’s certain there is other life SOMEWHERE out there in the cosmos), and others would be how to achieve cold fusion or better yet some other way of producing energy without any waste products, and what Yankee Rose meant at the end of the Satanic Bible (LaVey took the secret to his grave).
I would determine if FTL travel is possible, and how best to go about doing it.
I would figure out where the closest human friendly planets are at.
I would envision a handful of winning powerball lotto numbers (and Mega Millions).
I would envision the stock market trends for the next 15 years or so.
I would figure out where the necessary resources are located for that FTL method.
I would determine the location of resources on the Moon/Mars valuable enough to make it worthwhile for folks to go.
I would spend the rest of the time sending y omniscience to girls locker rooms. HEhehe
What has Dick Cheney been up to for the past 8 years?
I would write one really bitchin’ limerick. Maybe two. With horrible puns in them.
Hey, it’s only five minutes.
I’d pick the winning lottery numbers for the next ten years for every state lottery in the country, and use them all every week.
I’d peek into OJ Simpson’s memories of June 12, 1994 to confirm what I pretty much know.
I’d see when certain people are scheduled to die. And I’d make sure to find out the 13 youngest of them to use in the next Celebrity Death Pool.
I would do that as well. The coolest part would be sitting back and laughing as the authorities attempted to figure out how I pulled it off.
Hey, I came into this thread to say that!
(Actually, I was going to say that I’d visit deadoralive.com and take notes…)
I’d NEVER tell anyone what I specifically focused on. It would keep everybody else guessing.
“That guy was omniscient for five minutes. Think he knows whether there’s a God or not? Who killed JFK? Where Google’s stock will be in five years?”
I’d figure out a way to safely enhance the size of the male genitalia. No need to look at stock prices if you got that info!
I’d spend the rest of the time watching my own personal historical porn channel.
Did you send me an email this morning?
Besides the lottery and stock picks, a lot, I’d also take a variant on the crushes thing–I’d know the exact things to say at the right times, if it were possible, to get all of my crushes on permanent friends-with-benefits status. Totally sweet.
As for cancer, if it’s not something I can jot down I figure my lottery winnings and subsequent investments will pay for a lot of research. =P