By no means do you need to be able to read sheet music. There tons and tons of famous/ talented musicians who cannot read music. Can you read guitar tabs? This might be a big help and is a lot easier to learn quickly than reading music.
That being said, being able to read music does come in handy if you want to involve yourself in instruments other than guitar (like piano, in which you kind-of have to be able to read music), or if you want to write your songs down in this format for whatever reason. I'm sure there are lots of books/ websites out there that can teach you how to read music. Buy a 'beginner's guide to music theory' type thing and see how much you can learn from it. But reading music is not an absolute necessity needed for forming a band and writing your own songs.
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