I live in Portland, OR and was referred to this site when I searched up online

First, just this week, I have been talked into hermit crabs, and really want to be an owner, but have no clue what to do.
So, bear with me with my questions, it's just that I am new and want to do everything right.
Here are my questions, I hope they aren't too crazy!

How many gallons does my tank need to be?
What kind of dirt do they need, or is sand a better choice?
How many can I get in my tank?
What water bowl do I need to get, I do know that they need a freshwater and a saltwater?
How do you suggest I set up a new tank?
Ok, that's all for now, I really need help!