I was wondering which is the correct form. радио or ра́дио ? I mean I've seen people writing both of them. And I've also seen many letters with this accent above the a. Is it necessary? Which spelling is correct?
Both forms are correct.
The "accent" is a stress mark. Stress marks are omitted in most books. They are printed in books for beginner readers, and in words where a change of stress would change the meaning.