So you passed the interview, negotiated a salary you are happy with, and are leaving your current position for greener pastures. It is time to coast for the next six months or so when people stop thinking of you as the newbie.
New job cookbook
Dress as comfortably as you can get away with
Dressing purely for comfort and not caring what people think of you is not professional. Unless it would be totally out of place i recommend dressing at least as nice as the best dressed person you encountered while interviewing. Some people may have been walking around the office in shorts and flip flops but unless that is how everyone is dressing that is not a good tone to set. If you are not sure then error on the side of dressing up rather than down. No one will take note if you start dressing more casually later but if randomly you start coming in a dress shirt and tie people prepare for the half joking questions asking if you have an interview later.
Find a way to prove yourself to the team and show them that is was good their good fortune to have hired you.
They offered you the job and wanted you to join the team. You have nothing to prove. You have already proved it. Looking for a way to make a mark day one will just add stress and get in the way of learning your new work ecosystem. Relax. Build relationships and get a lay of the land. Only then can you carve out our niche and add value.
Do not ask for help. That will make it seem like it was a mistake to hire you.
You are new and just keeping the lay of the land you should be asking questions to speed up the process. People also enjoy feeling useful and helpful. A question can be a good icebreaker and an opportunity to set the tone for the relationship with your new colleague. Be open and honest about what you don't know and you will be surprised how willing people are to help.
Stay in your cubicle and wait for people to see you out
To be productive at a new company you cannot be an island on to yourself. Even if it is against your nature you need to get out there and meet people. Do not have lunch at your desk. Seek out opportunities to meet people. A good way to do this is sitting in on other teams stand ups. Being a fly on the wall for stand ups is a great way to meet people outside of your immediate team and increase the bredth of your knowledge about the company. To be an effective member of the team people need to know you and you need to make an effort to know them. For the first couple weeks i like to keep a text files with short bios about people to help me keep track of things and not get overwhelmed.
There are always a few key people to know that might hold no real power or authority but are the keys to the social network of the company. For example always be nice to the receptionist, office administrator, and cleaning staff. Just because a person has a less glamorous job than you doesn't mean their position isn't vital to the day to day activities of the company. You would be surprised how many hurdles can be overcome just by being nice to the unsung heroes.
Criticize how things are done and offer better ways of doing things
Before you start offering unsolicited advice and criticism learn the details as to why things are that way. Decisions are not made a whims and likely at one time there was a reason behind the decision that someone felt was a good one. Even if you don't agree and everything seems totally wrong hold your tongue until you learn how they got to be this way. Once when i started a new company everything seemed to be anti-pattern and opposite of the way the industry was headed but it would have done me no good to point that out. The people maintaining the code base built it years ago and were very proud of it. You would not criticize someone else's child and likewise you should not criticize their code. Learn how things are and slowly make suggestions and advise who to course correct but only after you understand how things got to this point. Starting off criticizing the way things are done and showing people a "better" way of doing things will just make you seem arrogant.
Never talk to your new boss unless spoken to
People are busy and more than likely your boss has more one her plate than just your onboarding. Make a point of touching base regularly. John sonmez from www.Simpleprogrammer.Com recommends sending a weekly status report. The status report does not have to be detailed and can just be a bulleted list of what you have done. These weekly status reports can be a nice way to stay on your bosses radar and provide documentation for future discussions around reviews and why you might deserve a promotion. Starting this habit week one will show your new boss that you are not a typical employee and you will make their lives better by having you on the team.
You know the technology so you don't need to understand the business to be effective
The technology you are working on is often the easy part. There are books, blog posts, conferences, and webinars to help you acquire what ever technology you need to learn but there is rarely a book out there about your company. If you work for an insurance company you should learn about insurance. If you work for an financial services company then you certainly need to learn about investing. In the vast majority of cases technology is a means to an end not the product itself so you need to learn business domain you are a part of so that the technological decisions you make are in tune with the bigger picture.
Assume your org chart is an accurate reflection of reality
The org chart may tell you the reporting structure according to hr but that rarely defines how things actually are. Take note of the people who are either friendly with those in charge and the people who teammates fear crossing. A persons may not have a fancy title but that doesn't mean that they are not a trusted adviser. Not all people seek to climb the corporate ladder so it is quite possible the rock stars who drive company do so happily from the shadows because they enjoy where they are at. Most importantly know who you do not want to be mad at you. There are often a few people around who are in everyone else's business and can make your life difficult if you cross them. I know it isn't how it should be but there often people you just have keep a wide berth of and try to being friendly with because they will make your life miserable if you don't.
Wrap up
Clearly this was a recipe for what not to do. Your first week at your new company will set the tone for your tenure there. The impressions that people make of you will be difficult to change later. Sure nothing is set in stone and you can turn things around later but a little effort can help help you start off on the right foot. Determine how you want to be perceived and take action to present that to your colleagues.
Don't coast through your first few weeks at a company. Use that time to build a foundation for your future there.