The adventure log is where you list the sessions and adventures your party has been on, but for now, we suggest doing a very light “story so far” post. Just give a brief overview of what the party has done up to this point. After each future session, create a new post detailing that night’s adventures.
One final tip: Don’t stress about making your Obsidian Portal campaign look perfect. Instead, just make it work for you and your group. If everyone is having fun, then you’re using Obsidian Portal exactly as it was designed, even if your adventure log isn’t always up to date or your characters don’t all have portrait pictures.