If you will stay in Sweden for one year or longer one of the first things to do when you have arrived is to go to the common service office for the Swedish Tax Agency, the Swedish Social Insurance Agency and the Swedish Pensions Agency. You have to go there in person in order to apply for a personal identity number. This is also the place to turn to in order to get an ID card, to ask about tax related matters, social insurance in Sweden and, when it is time to leave Sweden, issues related to pensions.
At the service office you will also get a form from the Swedish Social Insurance Agency so you can register for social benefits in Sweden. That registration can be done when you have got your personal identity number.
Please note that if you have brought your family with you, they also need to come in person to register at the service office.
Information about services at the Swedish Tax Agency
Information about the Swedish Social Insurance Agency
Information about pensions