How do I dry my foot cast when its wet?

It should not be wet. You need to call your doctor. Depending on wet it is, it may need to be changed. A wet cast can create a fungus infection of the skin and start to smell.
Try a hair blower. Place it on the cooler setting rather than the heat so you don't burn yourself. They do sell shower cast covers - prevention is always best. You also need to inform your doc as a wet cast may be problematic.