Here are my opinions: In all respects except one, I agree with try2win. I do believe that now is a fine time to invest - I do not agree that you can or should try to time the market (that is what try2win is indicating). So, you could start investing now, but as try2win says, you need to gather some more cash and consider what you are investing for before buying ETFs.
Right now, you must plow any available cash that you have into your 401K/IRA and use broad market (For your age I suggest US 40%, INTL 50%, EMERGING MKTS 10%) - that would be 3 broad mutual funds with low expense ratios (Vangaurd is the best). If you are going to school and working part time or are self employed, open a ROTH IRA at Fidelity and max it out. Use Vangaurd mutual funds if you can.
After you have totally maxed out your 401K/IRA and want to invest more, that is the time to start considering ETFs.
If you really want to do some "smart" gambling in the market (instead of blowing $200 at a casino or a bar), play with options. You may get lucky and be a big winner. The odds are stacked against you but you may find it fun and you will definetly learn a lot. I actually got very lucky and made a million or so in the 1990s. But you need to always remember that you will most likely lose any money you invest in options - but IMO the odds are way better than at the casino! and man-o-man that is fun!
Anyway - before you play with any hard earned cash, max out your yearly retirement account(s) FIRST.
Note also that there is a school of thought (and some compelling scientific data) that would indicate that at your age that you could actually borrow money, invest it in the broad world market, and make a killing - all because your time horizon is so long. You could easily buy and hold an investment for 40 to 50 years! Old guys like some of us that have a lot of experience can not consider that. (Why is youth wasted on the young?!) I wonder if it is legal to get a student loan and buy ETFs with that...
(BTW, if you own your own company, invest all you can in yourself, those returns can be really great.)
So, max out your retirement accounts in your 20s and you should be a multi-millionaire in your 50s. And, don't forget to eat right, exersize, and do a good deed - everyday!
Good luck!