"Start now. It's a lot easier to save now than it is to start to save later."
Start as early as possible and you will want to kiss your younger self when you get to retirement age. I know you (and everyone else at that age) thinks that they don't make enough to start saving and leans towards waiting until you get established in your career and start making better money. Don't put it off. Save some money out of each paycheck even if it is only $50.
Trust me, as little as you make now, you probably have more disposable income than you will when you make twice as much. Your lifestyle always seems to keep up with your income and you will likely ALWAYS feel like you don't have money left over to save.
The longer you wait, the more you are going to have to stuff away to make up for that lost time you could have been compounding your returns.
Open up a Roth IRA at a discount-broker, like TD Ameritrade, invest in no-fee ETF's, tracking an Index, with very low expense ratios (look for around .15%) This way, you won't pay brokers fees whenever you buy shares, and shares are cheap enough to buy casually.
This is a good way to start. When you learn more about the market, you can check out individual stocks, exploring different market sectors, etc. But you won't regret starting with a good index fund. Also, it's easy to know how well you did. Just listen on the radio or online for how the Dow or S&P did that day/month/year. Your account will mirror these changes!