If you are not sure how to sit and how to breathe — these instructions (one page) from the Zen Mountain Monastery in upstate New York are very clear and complete. The photos are helpful too.
If you are searching for a cushion, check these two sites: DharmaCrafts and Samadhi Cushions. I have sat on cushions from both places, and the quality is nice. But you don’t need to buy anything. You can sit on an old phone book, if that works for you.