I met my friend.
>This sentence is correct. Good job!

I know how to play thid game.
> Do you mean "This game?". This sentence is correct as well.

On Christmas I wish I get a lego.
> On Christmas, I wish  I could get a Lego. 
> Could is used to express possibility as well as to make suggestions or request.

In my house I have little car toys.
> If you are referring to the sizes of your toys, you can also say:
~I have small toy cars. Small and little could be used in describing a size but small is more common. 


"You have to study." said my mom.
>This sentence is correct. 

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Thank you so much Eric for doing your homework.  Here are your corrections and some notes that I made.

Take care always and take time to read your English books. See you on Monday and have a wonderful weekend. ^^