A sweet sister

My sisters' name is call Valentina, she is have fifteen years old and soon she'll begin the university., She doesn't yet  don't know yet what to study, but I think that she can should study journalism because she loves talking with the people about everything and anything., Even if she writes horrible badly and she don't doesn't like reading, I hope that she'll be the best at whichever career she chooses. anything career she choose, she'll become in the best.

A sweet sister

My sister's name is Valentina. <-- Your sister's name belongs to her, so use 's to show ownership. End your sentences with a full stop.

She is fifteen years old and will be attending university soon. <--  We use the present indicative tense of the verb to be when talking about someones age. Also, you don't need to include the subject noun (she) twice in one sentence.

She doesn't know what she wants to study yet or She hasn't decided what to study yet,

but I think that she should study journalism because she loves to talk to people about anything.

Even though she writes horribly <-- 'Horrible' is an adjective that is used to describe nouns. Since we are describing the way she writes ('write' is a verb), we need to use an adverb.

and doesn't like to read, I hope that she will succeed in whichever career she pursues. <-- You don't need to include the subject noun (she) twice in one sentence.