Now! I wish I would have started investing sooner. A job might come with a 401k, but you might consider a Roth IRA. A Roth is an investment tool where you contribute to the fund with after-tax income but any withdrawals you make down the road are tax free (up to what you've contributed until you reach retirement age). The trick on the Roth is you can only make contributions to it under a certain income level, which makes it ideal for a young investor. Bonus with the Roth? Because you've already paid taxes on what you put in, you can withdraw from it at ANY time - like when you buy a house - with no penalty. So, if I started contributing to my Roth in college, let's say I ended up putting $10k in there before I earned enough income to not qualify, and earned another $10k in interest. When I bought my house, I could draw from it up to what I contributed - $10k - towards my down payment without paying ANY taxes! If I pulled from my other retirement account, I'd have to pay taxes. Womp womp.
Here's an easy tool to learn more: http://www.rothira.com/
I agree with Jewelyn - a ROTH IRA is a great way to start. In fact I took an accounting 101 class my sophomore year where one of the assignments was to open our own ROTH! I've had that account ever since (10+ years). It's important to remember than when investing with little prior experience not to get too crazy. How much risk (i.e. How much you can put in stocks and other riskier investments) depends on your willingness to tolerate potential losses and your ability to take risk. Since you're young and have a long way until you retire, you are more able to take risk, however make sure that whatever you put aside is not money you will need right away. Do your research first and since your just beginning, look into low-cost index funds or ETFS. The key isn't necessarily what you buy but that you consistently set aside money to invest and make sure it's rebalanced to fit your risk profile about once a year.