New Post: Pricing from Options

I have a site that sells tennis lessons. I would like some help to figure out a few things that I hope are ultimately small.

1. Filling in the first option text box should be 'required' to purchase a product.
2. Have the 'Buy' button visible even though the price is set to 0 so that the purchase price is set from the options drop down list.
3. Change the 'buy' text to 'purchase' on the buy button.
   

Can someone help me get to the right place(s) to make these changes?