US citizenship

Hello:

I have been a greencard holder for more than 4 years and I have read postings in this site about when a person can apply for citizenship. I have a few questions hope someone can give me some help.

I. When can I apply for citizenship?
II. After I applied, how long do I need to wait to take the test?
III. After how long I can get my citizenship?

I am in California.

I really appreciate your help.