Can I only get tetanus if I step on a rusty nail?

Any wound a problem. Tetanus spores from the dirt can get into any break in your skin and potentially germinate. Over time the growing germs may elaborate the tetanus toxin and give you tetanus. The rusty nail scene is the one most talked about, but any dirty wound in a susceptable person can be a source.
No. Tetanus spores are commonly found in the soil, and the disease has been associated with wounds contaminated with dirt, feces, saliva. It can even occur in apparently clean wounds, which is why it is so important to keep your tetanus immunization up to date. It can't be transmitted person to person.