。
Yumi will probably become a teacher.
Literal
Yumi [topic-は] teacher become-[conjectural-でしょう].
先生になる ('to become a teacher') uses ~になる for a change of identity. でしょう hedges the statement as a prediction rather than a certainty. Note how Japanese doesn't distinguish 'a teacher' from 'the teacher' — no articles — and will/probably-will is expressed through でしょう.