Summarize this article:

If you live in the United States, you'll notice blue USPS mailboxes in most cities and suburbs. Open the box near the top, place your letter in the slot, and close it. A postal worker will collect your letter at the time shown on the box and start the sorting-and-delivery process. Every blue collection box has a notice indicating when the mail gets picked up each day. If you place your letter in the box after the scheduled pick-up time, your letter will be picked up the next business day. If you have a mailbox on or near your home, you can place your letter there. Alert your mail carrier of its presence by moving the red flag out of the mailbox.  This is usually done by rotating the flag up or by pulling the flag out.  The mail carrier knows that if the flag is visible, then there is a letter that needs attention. If you need to buy postage, you can leave your letter with a postal clerk, and s/he will mail it for you. Even if you don't need to buy postage, you can still take your letter to the post office to be mailed. See www.usps.com for information on where to find the closest post office.

Summary:
Place the letter in a blue collection box. Place the letter in your own mailbox. Take the letter to any post office.