ください

ください is a Japanese word meaning “please”, however it is commonly used for commands (see below).

をください

Give me noun, please
Nounをください

 ください is a special verb that means “please“. It can be used with を to ask for a noun. 
を may be dropped. 

You may also hear this as ちょうだい. It has the same meaning but is (usually) less formal.
を is also commonly dropped with ちょうだい.

Examples

• リンゴをください。- Apple, please　→ Please give me an apple.
• えんぴつを3ぼんください。Pencil, 3 please → Please give me 3 pencils.
• しおをください。Salt, please → Please pass the salt

Verbてください means please do something. 

Examples

• この本を読んでください。- Please read this book. → Read it.
• 待ってください。- Wait please → wait.

ないでください

Don’t do X (please)
Verb ないで +ください

The negative of てください is ないで ＋ください

• それをしないでください。- Don’t do that.
• 見ないで – Don’t look.
• 食べないでください – Don’t eat (it). 
  
  
  

As a command

Although ください means please, it is more typically used as a command. This is similar to “requests” in English such as “Please wait your turn” at a crowded venue, etc. These are not really requests, but instructions. 
Command ください is very common, and typically doesn’t drop ください. It is basically the only option for a polite command you have with non-friends in daily life, without bringing social status into it. 

See also Commands for other command forms and why you shouldn’t use them. 
Reference (Japanese)

Examples

Imagine your teacher says:

• この本をよんでください。- Read this book, please
• 46ページに開けてください。- Please open to page 46.

The teacher is nicely telling you what to do. 

With よかったら

てください can be made into a explicit or softer request by adding a word like よかったら at the start of the sentence. This is usually used when there is a choice. 

• よかったら聞いてください – If its good with you, ask/listen please
• よかったら食べてください – If its good with you, eat it please.
• よかったらなかで待ってください – If its good with you, wait inside please. (You don’t have to)

See also ~てほしい