Is or are

No. The subject is 'focus', so 'is' is the correct verb.

Thanks. But the focus is on two things, exercise and good diet. So I am wondering if there is a plural form for focus, just like my goal is, and my goals are. Would a teacher please help?

The verb is determined by "focus", not by "exercise" and "diet".

Then what about the other example, my goal is and my goals are?

What is the difference? We usually use my goal is when there is one goal, and my goals are when there are two or more?

My main goal is studying hard and doing well in my exams.

My goals are doing well in the exams and getting a place in a university.

Your first sentence has a consequence of a main goal, so I understand that it is possible, but it is seen as a result and an ultimate consequence. It could have be written also: my main goals are studying hard and doing well in my exams. Basically depends on the way you see it.

But I would like to know if there is a plural form for my original sentence with the 'focus', just as how you in

No. "eating good food and exercising hard" is the focus, not focuses.