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Pay attention to the keywords: convince (give reasoned arguments), a friend (informal style, you can use contractions) healthier (that means that your friend already follows a healthy diet so you have to suggest how to improve it), and diet (only his/her diet, not lifestyle).
To review writing a composition, have a look at these two links: Composition structure and Composition Writing: the main stages.
I've found this video that also give you some ideas:
Have a nice weekend.
