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

Two, and a puctuation error. Corrected, it is: You'd (or You had) better speak to John if you need some help with physics (no capital p), then either a period or a semi-colon instead of a dash. If you use a period, capitalize "You". but if you use a semi-colon you don't need to.

I answered this question fully earlier today. see https://answers.yahoo.com/question/index?qid=20170...

Are you trolling? Wasting our time? Or genuinely not interested in the correct use of English?

I described many problems.

I won't do so again.

"It would be better if you spoke to John about your English. You know that you are not very good in the correct use of it."

Correct sentence will be

You better speak to John if you need some help in Physics- You know that I am useless in physics.

Use in in both places. Physics is a topic not an object to use with or at.

You HAD better speak to John...

or

You'd better speak to John....

I'm, not i'm.