This is notoriously confusing, because English and Italian use opposite sentence structures to describe the feeling of missing someone.
In English, this is rendered as a literal feeling. The subject of the sentence is the person who is feeling the absence of the other person. The other person is the object of this feeling.
In Italian, the focus is on the person that is absent: they are the subject of the sentence. The person who is feeling this absence, on the other hand, is indicated with a complemento di termine: with the “a” preposition, or with mi/ti/gli…
Mi mancherai (tu mancherai a me) means “I will miss you”. Imagine that this literally translated “You will be away/absent/missing from me”.
Ti mancherò (io mancherò a te) is the opposite, so it means “You will miss me”.
*Ti mancherai (*tu mancherai a te) is usually meaningless in Italian, as it would translate as “you will miss yourself”.
Mancherò is meaningless if not accompanied with some extra information (mancherò al lavoro: I’ll be absent from work; mancherò alla mamma: mum will miss me).