Unless you really want to play with buying/selling individual stocks, you're probably best off buying a broad-based index fund, which effectively buys you a slice of the entire stock market. Eg VTSMX/VTSAX
Also, are your 401k and IRA/Roth fully funded? Be sure to do that first. (Assuming USA resident here)
To add to Nick's point, you can read Random Walk Down Wall Street to get an understand of why buying an index fund is probably a good idea for you. It boils down to the fact that the market is fairly efficient "most" of the time (hard to find deals), and by trading you are already at a disadvantage against the market because you will likely pay more commission and pay more taxes (based on your trading volume of course, but generally speaking, buying index funds are extremely efficient in both these two areas). To over come this and still beat the market as a whole (over a long period of time and risk-adjusted) is a extremely difficult. You have to be very very skilled to accomplish this. Most pros do not accomplish this in their careers (even though it may be due to other factors such as incentive structures, but that's for another discussion). So realistically speaking, you will have to invest a lot of time to become "good enough", which means it will only be worth while if you are trading a significant amount of money. Even Warren Buffett, who's an evangelists for value investing recommends investing in index funds for most people. And the last and most important thing to remember is that yes, people make money from the stock market all the time just like how people make money from the casino all the time. It doesn't mean it makes sense risk-adjusted, and that it can be done over a long period of time.
Could you make an argument for bothering to fund 401k + IRA/Roth at 20-something? If I'm trying to get fuck-you money by 40, don't I need every dime in order to get there? My nest egg is in plain old savings right now because I view it as personal runway.
If you plan on being alive at 65, then yes you should. Both are fantastic investments because of the tax free growth. Basically everyone should follow this pattern with extra money they have:
1.) Max out employer matching in 401k
2.) Max out roth IRA contribution (in most cases, sometimes you might want a normal IRA)
I don't understand your implication that striving in a career endeavor requires that you spend all your income with nothing going to savings. If anything, healthy savings will allow you to take advantage of an unexpected opportunity, and/or lessen the impact of an unexpected obstacle. This 'backup plan' will also make it easier to take risks without having to worry as much about the potential for failure. In other words, savings provides you with options.
But that's in terms of general savings. For retirement more specifically, you should definitely start now if you can afford it, due to the effects of compound interest. Try playing around with an interest calculator[1] to see why. But perhaps the best reason is that once you're 40 and have had a few curve balls thrown at you, you'll appreciate that regardless of how things turned out, you'll have put yourself in a good place financially for the long-term.
For the record, I'm in my mid-20s myself, and across my Roth and my employer's SEP-IRA, around 20% of my income is going towards retirement (with other non-retirement savings on top of that). I've set up my direct deposit such that the Roth contributions are totally automatic. It just gets dumped into three index funds: US Stock, Intl Stock, and Bonds. I basically never need to think about the accounts except when rebalancing. It's nice.
If you can't afford to fully fund your 401k without thinking twice, you have bigger problems in your financial setup. $16,500 out of a $100k yearly salary is just stupid cheap.
If you are not making $100k+, you are not going to have fuck-you money by 40. All the Facebook stock in the world still requires a liquidity event, which is unrealistic at the current valuation.
Also, are your 401k and IRA/Roth fully funded? Be sure to do that first. (Assuming USA resident here)