Easy 3 day detox diet to clear out body

I’m 16 and healthy, but I’ve been eating a lot of rubbish due to lack of time to prepare food. I can feel loads of ‘junk’ in my body and want to give my body a break and let it cleanse its self.
I thought about a water fast for 3 days with a 30 minute walk, would this work for getting rid of impurities?

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