I personally have everything in the S&P 500 index fund. The fund gives you diversity, at least when it comes to stock ownership across the spectrum of large American companies. In addition the expense ratio for the fund is very low so you keep more of your money.
If you are less than 40 years old and not making a ton of money, I would strongly encourage you to go with the "Roth" option. While you will not be able to deduct the contribution from your taxes, all of the money you make (your capital appreciation and dividends) will be completely tax free so long as you keep your money in a Roth 401k until age 59.5.
UpState made a good point in his post. Do you have enough income to be able to stash away some percentage of your earnings until age 59.5. If you are looking at saving up for a downpayment on a house, for example, you may want to hold off on a 401K in order to pursue that goal first.
Also, keep in mind you can contribute to a Roth IRA, where the rules are quite similar to those of a 401k. The difference being that the IRA is not through your employer and you simply write a check, up to $5000 per calendar year, for how much you wish to invest to the institution holding your money, hence the name Individual Retirement Account. If I were not parking at least the $5000 maximum into an IRA, I personally would want to fully fund my IRA before seeking to park additional funds in a 401K since UPS does not offer any kind of match to the 401K. For my IRA, I personally have an account with Vanguard. In short, I have found this to be an outstanding company with which to do business. vanguard.com
Another thing that I have found helpful when it comes to investing is to have a strong stomach. The market goes up and it goes down as well with the overall trend being up over long periods (decades) of time. Some people performance chase, meaning they tend to buy as the market is approaching a peak, and then, when it goes down, they panic and sell at a loss.
Hope all this helps you...In a rather recent post I tried to encourage someone with a hymn which had recently been sung in church...instead of being encouraging, it seemd like I ended up completely confusing the person since he did not appear to have the backround knowledge to properly understand the text. So I hope I have done a better job here at providing useful and understandable information.