I’m a Bachur going to a Shliach for Rosh Hashana. If I will be Chazan whether Shacharis or Musaf should I wear a Tallis? And if yes, do I make a Bracha or not?
You don’t need to wear a Tallis.
If the congregation is particular about this, you should wear it just over the shoulders (together with a hat), so it shouldn’t seem fully as a Tallis Gadol. In this case, you should use a borrowed Tallis – not a communal Tallis so you won’t need to make a Brocha.
(Regarding Selichos, and Mariv and Neilah of Yom Kippur – where Yud Gimmel Middos is said – see #5657).