私 means “I”.
魔女 means “a witch”.
にはなりたくない: If I break this down, it’s になりたくない with the particle は.
Noun になりたくない means “not want to become” something. なりたくない is the negative form of なりたい meaning “want to become” which consists of なる meaning “to become” and the auxiliary verb たい meaning “want to”. So 魔女になりたくない means “not want to become a witch”.
Then why do you need this particle は? – は particle basically has two functions. One is for the topic of a sentence, and the other is for contrast. In this sentence は for the topic is supposed to be after 私 but it’s omitted here which often happens in colloquial speech. So this は is the particle for contrast. In this situation, she is contrasting “being a witch” with “not being a witch”. Using this contrastive particle は changes the nuance of this sentence to “I want to become anything but a witch”. So she is implying that she really does NOT want to become a witch.