Savings/401k question.

So I just accepted a job offer in the eastern Ohio area, this is my first "real" job out of the college. The job only pays $39,000 / year before taxes and I wasn't quite sure what is the best thing to do with money leftover after my core expenses.

My rent + utilities are $850 / month.

Food costs about $200-$250 / month.

After these essentials, my take home pay will be about $1,400 / month.

How much of this should I put into the 401k my company offers? I've heard 15% is good. And what should I do with the rest of my take home pay? I currently have a Schwab Brokerage account and have about $3,000 all in a Vanguard Index fund, I could buy more of that each month I suppose.

Other side notes: I don't have any debt, and am still on my parent's health insurance, and am fortunate I don't have to spend anything on transportation really (ride my bike to work, parent' car).

Any help is much appreciated!