how do I send stuff in the mail?

I'm not sure what the process is because no one ever taught me and I've been too scared to go to the post office to ask because I'm not good at talking to people unless I know what I'm supposed to say. I don't know if there's stuff I gotta do with the box first or how the label part works. I've just never done it before.

I just wanted to ask here if someone could explain it to me so I don't feel embarrassed for myself if I go to the post office and make a mistake.