I believe the default when not logged in, is to show the posts "Oldest first", as if it was your first time reading a thread and you wanted to start from the beginning. Once you join and log in, the default changes to "Newest first" based on the assumption that you have already read the earlier posts and now you just want to quickly catch up with what is new.
To change your preferences for how you view the threads when you are logged in so that you see the oldest first (as if you were not logged in), then follow these steps, which are illustrated with the screen capture links below:
1. Go to your "User CP"
2. Click on "Edit Options" and scroll down to the dialogue box for "Thread Display Options"
3. Select the display mode you prefer and then scroll way down to the bottom and click "Save Changes"
That should do it!