- Making mistakes will kill you, when you sit at home working on your personal projects and a bug slips through, it's not so bad, you can fix them and the client (or yourself) is normally pretty cool with it. On the other hand, when it's someone elses fault, E.G. yourself, your boss will come down hard on you. That's his project, his clients and his shame, if you make mistakes in a commercial environment, it costs someone money - get in to the habit of being your own quality assurance.
- Your wage is shitty. Well, mine is, anyway. You have to learn to deal with this, I could go in to detail about finding housing and the lack of respect you face when you need housing benefits, etc, or can't eat for a few days, but there's no need. Get used to having no money and always put as much as you can aside for that rainy day you think you are immortal from at the moment.
- Get used to being the go to guy for all the boring work, no one wants to sit there and refactor spaghetti code or build yet another custom joomla module when they have some cool CSS3 design or some technical challenge of epic proportions to work on.
- Give your boss respect, he may be friendly but he isn't one of your friends, you can joke with him but at the end of the day don't forget that this (wo)man pays your wages and evaluates your work, if you're making a lot of jokes but you have a week where your work goes wrong, the evaluation will be that you're the class clown.
- Slow down at the start, when everyone goes in to a new job, we work our asses off trying to impress then slow down once we settle in, even more so for a junior in their first role, we want to prove we belong in the industry. The kicker is that this pace is difficult to maintain and once you slow down your boss will notice your apparent lack of progress.
- Always be early, never be late. Your boss doesn't notice the fact that you come in early every day (by 15 minutes), what he does notice is the one day you're ten minutes late and unfortunately the hours aren't interchangeable. If you make a habit of being late, you are going to get fired, this isn't college where you could turn up to a lesson 15 minutes late and get a giggle from your peers for being that guy, this is the real world, where lateness isn't tolerated for very long.
- Arrogance is bad. You aren't arrogant, but you have an opinion and you come across as it, starting sentences with "actually, I think" or "No, that's wrong..." will land you in the shit, especially in front of a client. Your boss wants to appear to be an all singing, all dancing super coder and until you have a few more years under your belt, your opinion is largely invalid. However, if something is seriously wrong, drop your boss an email or a skype or a campfire (That's a thing, right?) and explain why in the least condescending way possible, if you do this tactically and only at the right times it will look very good on you.
If anyone is about to jump in to their first job as a junior or needs any advice, feel free to email me, I normally check it every couple of hours.