My today's topic is opus by Cervanpic today is Cervante's opus, Don Quixotes.

You could also say:

Today's topic is...

I say hHonestly, I didn't know to how calhe English titled, "Don Quixote" in English because I read it in Russian language.

Because of the context, you can drop "language" but if you want to keep it you need to add "the": in the Russian Language.

I thinkought this opunovel was called "Don Kihot" so my nickname is "kixot".

"Opus" isn't very widely used. It refers to an author's most important work. Using it once to introduce your feeling that this is a great work is fine, but it you keep using it, it feels a little unnatural or possibly very academic. You could used novel or book.

Also here change the tense to past tense. Yesterday I thought the novel was called. But today I don't think that.

We properly knew that II really like reading this book.

I think you want to give emphasis here so I added "really" but removed "We properly knew" as we the readers don't know.

I want to tell you my thoughts about the character Don Quixote.

He is a crazy man but I like this character because I find in him my own bita little similar to myself.

In opus by Cervantes' book, people think Don Quixote is crazy, but he doesn't worry about it.

He just goes his own way.

My today's topic is opus by Cervantespic of the day is Cervantes' opus, Don Quixote.

When I first read this sentence I thought the title of the work was "opus". Usually if you say "<x> by <author>" then <x> is the title of the work.

I say'll honestly say, I didn't know to how calledto pronounce "Don Quixote" in English because I read it in Russian language.

I'm assuming you meant "pronounce" here. You could also use "say".

I thinkought this opus was called "Don Kihot" so my nickname isbecame "kixot".

We properly knew that I liked reading this book.

Not sure what word you are trying to translate to "properly" here but it doesn't really fit well. I will defer to other reviewers on how to fix this

I want to tell you my thoughts about the character Don Quixote.

He is a crazy man but I like this character because I findsee myself in him my owna bit.

I'm assuming you want to say he is similar to you. There are other ways to say this, what I wrote is just one suggestion.

In opus by Cervantesthis book, people think Don Quixote is crazy but he doesn't worry itpay them any mind.

You could also say "he doesn't let that bother him"

He just godoes his wayown thing.

This is a casual way to say it. There are also more literary/poetic ways to word this, like "he chooses to forge his own path", etc.