Is beginning a pronoun

No, the Word 'beginning' is the present participle, present tense of the verb to begin. The present participle of the verb also functions as an adjective and a gerund, a verbal noun.

Examples:

I am beginning my training at the community college next month. (verb)

The classes for beginning swimmers are on Tuesday. (adjective)

The beginning of the story gets your attention. (noun)

A pronoun is a Word that takes the place of a noun in a sentence.

Example: The beginning of the story gets your attention. It has a lot of action. (the pronoun 'it' takes the place of the noun 'beginning' as the subject of the second sentence)