Each PC will keep his or her own Chronicle. During your initiation, this will just be a diary to keep track of your character as the time passes. A typical Pathfinder's journey to Confirmation takes 2 to 3 years.
At Confirmation, the character will receive a leather-bound journal, and this is where your character's chronicle begins. Each player will keep a record of their activities in their Chronicle. After each post in your Chronicle, you will make a copy and send it off to your venture captain. He/she may or may not reply. In addition, if you have joined a faction(and I highly suggest that you do), you will write a second letter to your faction contact in private text. These letters will help ensure your patronage within your faction, and the support and leads that you will receive from the Pathfinder Lodges across Golarion.
As fame and eternal glory in the Chronicles is one of the primary reasons to be a Pathfinder, it behooves you to make yourself sound good in your reports. Be careful not to take too much credit, as reports are often checked against each other, and discrepancies are corrected with a firm hand.