Hello, this may be a strange question, but I've just moved into some apartments and the mail boxes look similar to the mail boxes at a post office, I need to have a letter sent out in the morning and I don't know how to use these mail boxes for sending things out. The office for the apartments is closed so I can't ask them, and the neighbors are very.. unfriendly.. Please help =(

I would not trust the carrier to pick up a piece of outgoing mail from that mailbox, they may see it, they may not, I myself, would take it to a mailbox and mail it.

The mail is picked up when they deleiver mail to the mail boxes, in the post office mail boxes, they normally pick up several times a day, there they will only pick up when they deleiver the mail for the apartments,
In my apartment, that pick up is from 1 pm to 4 pm so that is when the letter is picked up. So obviously it will not go out that day from the actual post office. If it is something that has to be mailed and somewhere a certain day, take it to the local post office.

Carry it to a public mail box or the local post office.