Hi Paasche and welcome to WAP.
Stephie has given you some excelent advice. Like she said, just be yourself. They are interested in every little thing that you know or do. You are their eyes on the world. I hadn't thought of it, but I think she is right about sticking close to your own age. At least that is what has worked for me.
Personally, I like the thought of a PO Box, because you never know who might be looking over a pen pals shoulder. But that is a decision for you to make.
Happy writing. "-)

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Live simply, love generously, care deeply, speak kindly.

Cowards are cruel, but the brave love mercy.