What should I do on Tisha B'Av if...?

The best thing to do in this situation is to ask your Rabbi. Most of the people on Yahoo Answers (including me) cannot paskin, they can only tell you what their Rabbis have told them, etc. None of us know what community you come from, or what your hashkafa is, so our answers may be quite different from the answer you would get from your Rabbi.

That said, I am modern orthodox, and I will tell you what I believe the people in my community would do (although I don't know what my Rabbi would say):

Regarding chairs - before chatzot, try to avoid sitting for any extended amount of time, and if you must, sit on one of your lower, more uncomfortable chairs. If you are able to, sit on the floor when you can. After chatzot, you may return to sitting on normal chairs, so this will not be a problem (find out exactly when chatzot is where you live).

Regarding reading - it all depends on the content. As a general rule of thumb, anything you do on this day should help you to focus on the mourning. If it does not relate directly to any time of mourning in Jewish history, it should at least reflect the general tone of the day. Anything with lighthearted or comedic content is not appropriate for the day.