A. A person who acquires a new home or garment says the Shehecheyanu blessing, as does one who eats a new fruit or does a mitzvah for the first time (Shulchan Aruch, Orach Chayyim 223:3-4, 225:3). A kohen beginning his service in the sanctuary says the blessing because of the new elevation in his status (though other views explain his blessing differently).
The Chatam Sofer (Responsa Orach Chayyim 55) rules that a Bar-Mitzvah who fulfils the mitzvah of tefillin for the first time should say Shehecheyanu, thinking of all the subsequent mitzvot he will carry out.
The Radvaz’s ruling that a convert should recite Shehecheyanu on being accepted into Judaism is explained by Rabbi IJ Weiss on the basis that if a single mitzvah merits a Shehecheyanu, surely this applies to the beginning of a lifetime of Torah and mitzvot (Minchat Yitzchak 1:129).