What would cause air bubbles in the toilet when I'm showering?
Sounds like the main drain for the bathroom may be restricted. 

As you shower the water doesn't drain away fast enough, and the plumbing begins to fill with water.  As the pipes fill, air may be forced out through the toilet trap causing "bubbles".  Since the tub is the lowest drain, the water backs up there first. If you continued to shower, you'd eventually see the toilet overflow as well (if the tub edge is higher than the toilet rim). If the tub was deep enough, you'd eventually see the sink overflow too.

If you look at this crude representation of your bathroom, you'll be able to see what I mean.



Now, if we add in a clog we can start to see what happens when you shower.



As you shower the water cannot pass through the clog fast enough, and the pipes begin to fill.



At this point, any air trapped in the line between the drain and the toilet trap will be forced out and bubble up trough the trap.



As you continue to shower, the tub is the first to back up.



Eventually, if you shower long enough the toilet could also overflow.



Depending on the height of the sink, quite a bit of the bathroom would have to fill before it overflowed.



The first thing I'd try is to clear the drain using a Plumber's snake. A hand auger would likely work, and can be purchased at a local hardware store fairly cheaply. Drill powered augers are also available for a bit higher price, but require a bit more of a gentle touch (and obviously a drill).





Another option is that the vent for the bathroom is clogged, or not sized appropriately. If vent stacks are not covered properly, leaves and other debris can become lodged in the pipe and prevent air flow. A clogged vent will cause the fixtures it's venting to drain slowly, and the results can be similar to a clog as described above.

As with the clog, the solution to clear the blocked vent is a Plumber's snake.