The default order in which multiple posts are displayed in a thread is reverse historical order, with the newest post at the top. I haven't been able to find a way to change this for everyone. However, if you'd prefer the more logical (I think) order of oldest at the top with newer ones being added below, so that you can read the thread sequentially by sliding down it, rather than having to go down and up, down and up repeatedly, here's how:

1. Click on the User CP link at the left side of the blue bar in the top box on the page.

2. Click on Edit Options.

3. Find the box headed Thread Display Options.

4. Change the contents of Thread Display Mode from Linear - Newest First to Linear - Oldest First.

5. At the bottom of that page, click Save Changes.

That should do it.