I can't possibly understand why you would want to invest short term when you're young. Invest your money in long term items, like a house (if you already have a house buy a bigger one in a better neighborhood if you want serious ROI). Right now is the time to set up for your retirement and let your money do the most good for you. Definitely stay away from day trading. You would have about as much chance of making money playing blackjack.
The most important thing to do is set a goal. Say, I want to have 3 million in liquid assets at 60 years old. Then come up with an assumptive interest rate (maybe 7-8%) and find out what kind of investments will take you there. It's not that hard, and in fact there are websites (the fool, etc...) that will calculate that for you.
Now, if you just want to invest short term to raise more capital to invest long term, then by all means go at it, just don't be surprised when you get left holding the bag. If you do opt for an aggressive short term strategy, I would go see a CFP or broker for their advice.