How should invest my 401k contributions?

Your best bet is to invest in a Target Retirement date fund. It is based on your anticipated retirement date and they manage the fund for you, getting more conservative (more bonds) as you get older. Currently I have my account with Vanguard Target Retirement 2050 - VFIFX is the symbol. It's wise to invest your money in mutual funds and leave it stay put. If you want to do more active trading start another account for individual stocks and ETFs. Good for you for asking!