Mahou Shoujo Nante Mou Ii desu kara.

Yuzuka Hanami is a young girl who has few friends but who can do anything. One day, Yuzuka gets accosted by a strange creature named Miton who is digging through a trash dump. Miton tells Yuzuka, "you have the makings of a magical girl" and seems extremely suspicious. When Yuzuka is suddenly transformed into a magical girl, she's wearing a bathing suit.

(Source: Anime News Network)