Hi, I live in London and I need to get my citizenship from my late grandfather, he used to be a Brazilian Embassador in France, he went to Brazil and my mother was born there, afterwards they went back to France and lived there for ages [20 years almost] and now my mother lives in Brazil, I really need to get this citizenship before my visa expires. My mother also is not able to help me from Brazil as she doesn`t know how to get everything ready.

London would surely have an Embassy of Brazil so the place to start is there. Explain that your grandfather was an Ambassador to France. The Brazilian authorities would be able to certify his relations---your mother.
In that regard, you should propery identify yourself as the son of the said mother and prove that by getting birth certificates, hers and other siblings of her as well as siblings of yourself.
If such is not available, then prove in other ways. A driver's license showing the address of the mother and yourself as being one and the same.
Any other legal documents showing the sameness of your mother and yourself.

This can include tax receipts, home residence receipts of any kind such as taxes, utility bills etc.

Hi, I live in London and I need to get my citizenship from my late grandfather, he used to be a Brazilian Embassador in France, he went to Brazil and my mother was born there, afterwards they went back to France and lived there for ages [20 years almost] and now my mother lives in Brazil, I really need to get this citizenship before my visa expires. My mother also is not able to help me from Brazil as she doesn`t know how to get everything ready.

You need all your mother's documents... all proofs of relationship plus power of attorney from her. Normally the consulate won't response to you if you don't have a case. Contact the nearest French consulate with all the paperwork. Make sure you have a copy of everything.