How hard is it to learn to play guitar?

I’m a 13 year old girl and I am good at singing and like to write my own songs, so I want to learn to play the acoustic guitar so I have something I can play to accompany my voice. How hard is guitar to learn and how much does like, the average guitar cost? I can already play the piano, so I can read music.
Sorry if this seems like a dumb question but I don’t know much about guitars much.

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