Gabriele, this is great work!  Fret not! (Fret, if you don't yet know the word is a fancy way to say "worry."  So "Don't worry!")

Olympic Games

I highlighted a few errors, but look how few!  None of them will cause miscommunication.  Every word is understandable to the English speaker.

   1.  Because the Olympic Games is plural and technically the subject of your sentence, you would say The Olympic games begin on Friday, here in Rio de Janeiro.  Begin is only used in the third singular.  The way you said this is the way a lot of English speakers would say it, even though it is not technically correct.  So no actual problem there.

2.  world, and I.... you joined two complete sentences together with and (or any connecting word/conjunction) so use a comma where the first sentence would otherwise have a period. 

3.  English is not very good... Good is an adjective describing English.  Well is an adverb and has to describe a verb.  I don't speak Portugese well.  My Portugese is not very good.  (It is not!)

4. Even now I can express myself.  Use the present tense, can.  Could is the conditional so you might say "If I had a million dollars, I could hire a tutor to help me speak English."  You want "can" because yes, with or without a million dollars, you can express yourself in English! 

Nice work!