What are the most important things to consider with your 401K?

I’ve recently taken a “real” job and I have been investing in my 401k but its all still very confusing. Articles say I should be investing aggressively, should have saved a full year’s salary by now, and should be diversified between several different options. For the last year I have just stuck everything in one of those portfolios labeled 2045 (the year of my expected retirement). So what things do I need to consider and how much should I realistically be contributing. I still have student loans :(