The first thing you'll need are the rules to the game. You can read them in HTML form, but we are (slowly) working on other formats like PDF.
Once you are familiar with the rules, and you decide that Eressea might be the right game for you, you'll have to register for the game.
On the first Sunday afternoon after your registration you will receive an e-mail containing your initial report. It will contain the information about your starting position and it is the base from which you do your first turn. Reports come in two flavors: Normal report (.nr files, or just NR) and Computer reports (.cr files, or simply CR). The NR is a verbose plain text file, suitable for reading by humans. The CR file contains the same information, but it is not meant for printing - it is the data format that Magellan reads. Nowadays, most players use the CR in combination with Magellan (and some other tools).
Based on this initial turn, you will give orders to your units, which you will have to send to our server. The documentation tells you how to write the orders, and once you're done with them, send them to eressea-server@eressea.upb.de. Make sure your mail has the subject ERESSEA ORDERS, or it will not be considered.