Selenium when to use By id/name/class/spath/css, and page object

I start to work with selenium webdriver v2, i have a few qustions:

1- When to use By.id,By.name,By.className, By.cssSelector, By.xpath...

2- Did it good to combine all the By functions in the same test project?

3- When to use page object? did it recommendedto use for dynamic?