Meaning:

• よconveys the speaker's strong conviction or assertion:  "you know," "let me tell you," "contrary to what you think."
もう九時ですよ。起きなさい。It's 9 o'clock already.  Get up!
 
• よconveys the information the listener doesn't know.
左へ曲がると見えますよ。You'll see it after you turn left.
 

Location:  Behind just about any part of speech other than a question.

もう行きますよ。I'm going now.
あれは、桜ですよ。Those are cherry blossoms.
こいよ。Come!

Gender:  Both men and women use this particle.

Level:  Informal

Alternatives:

• Women may use わよ or の instead.  Go to no (の)
もう行くわよ。I'm going.
明日行くの。I'm going tomorrow.
 
• Men may use ぜ instead.  Go to ze (ぜ)
泣いたってどうしようもならないぜ 。Crying doesn't get you anywhere.

Warning: 

※よ should be avoided when speaking to a superior.

※When the speaker shares the same information with the listener, ね rather than よshould be used.