Is Castro District in San Francisco walkable? Can I walk to everything or do I need a car?

The Castro District is definitely walkable. There are some hills, but that's something you have to get used to in all of San Francisco. If you need to get from one end of The Castro to another, then you can always take public transportation. The J goes down Church St., the 24 goes down Castro St., and the 33 travels along 18th St.