is there any grammar mistake in this sentence? You better speak to John if you need some help with Physics- You know i'm useless at physics?

There are many problems. It isn't a complete or structured sentence.

"You better" A colloquialism at best. It cannot be improved by any mechanism.

"You should" is an improvement.

"You should talk with John." is a full statement or sentence.

What is the connector to the next sentence.?

"You know that I am useless at physics."

It isn't necessary as both sentences are valid. If a connector is used it must be appropriate.

Such as "You should take to John if you need some help with physics because you know that I am useless at physics"

Now the question mark is inappropriate.

Finally what is meant by " I am useless at physics"

It is also a colloquialism but is not a correct use of the term "useless"

" I do not understand physics"

"I cannot teach physics"

"I am unable to help you to understand physics"

Various phrases might carry a precise meaning about WHY you cannot help.

"You should speak to John if you need some help with physics because you know that I have little knowledge of physics"

As the last step you notice that the word "physics" has been used twice within the one sentence.

This is clumsy and tends to be frowned upon.

"You should speak to John if you need some help with physics because you know that I have little knowledge of the subject"

Of course there are many other ways of handling what you wish to say. But this is my suggestion of the best way to say it using the structure that you already provided.

I don't understand your question.

This belongs in English section.

No

Are there any...

Is there a...

sigh!