Some random thoughts - I'm a Systems Admin/Engineer by trade, but have spent some time in the Manager's chair as well. Here's some thoughts that would have helped me for my first real job (i.e. why didn't someone tell me):
Don't act like a know-it-all, but don't be afraid to offer a concise answer if solicited. Us nerds tend to LOVE to elaborate to "make an impression". The only impression is "Thanks, now how do I shut this guy up?".
Don't be afraid to challenge your peers when your approach may be better, but know when to back down when you're out-numbered.
In a cubical farm, your voice or other odd noises carry much further than you think. Same applies for the office restroom.
Speakerphone is a privilege reserved those with an office and a closed door. (Ab)using speakerphone (esp. loudly) in a cubicle will earn many more enemies than friends. Consider a headset if you're going to be on the phone a lot and need to type or do other things with your hands.
Learn and understand your company's core values. Chances are, you hold some of these yourself or you wouldn't have been hired (at least by any competent manager). These values will help guide your management team's decisions, just in case you question their motives.
If you lied on your interview, you'll be quickly found out the moment you submit your first program. If it's a serious lie that you can't lie your way out of again, don't even bother showing up for your first day.
Learn how to comment your code, but don't do it so much that stripping them out strips 75% of the file.
Use sane variables that someone else can maintain. "a, b, foo and bar" are not sane.
Be willing to learn - always. This may involve learning OUTSIDE of your job as well.
You will be required to understand the business to a degree that helps you develop useful code. Don't be surprised if your first few weeks on the job involve training that seems initially pointless (stocking shelves, packing boxes, etc.). It will all make sense in the end, and may even help start the creative flow of "I could do that better".
Innovate or get out of the way. Complacency often gets you fired.
For some companies, continuous improvement is expected. For others, they prefer the tried and true. Don't be afraid to ride the middle if you aren't sure which one you're dealing with at first. Someone will set you straight.
Make sure your line of communication with your manager is wide open. Understand his/her expectations and deliver on what's asked. However, if the expectations are completely unreasonable, have a backbone and ensure they know why you can't rewrite Linux in a day.
This is hard at first, but employees who perform well need little management. Those who don't can expect constant management. Then again, some managers like to micro-manage anyway, especially their under-performers.