This is one of the things that is very different for Americans, specifically. Mailing is a chore that many expats have their drivers or househelps handle, but you may need to do it on your own.
You cannot mail things from your house here. You cannot drop things into a mailbox on the side of the street here. You have to go to the post office to mail things.
So, the first thing you need to know is where your nearest post office is located. Only, hey, guess what? At the time of posting, the list of provinces you have to select from is actually missing several options, including one for the National Capital Region, where most expats who move here live. Maybe you live elsewhere, and you will be able to find your post office on the list. For the rest of us, we will need to ask our neighbors, or the people who run your village or condo should be able to tell you.
Second, you go down to the post office and get in line. There are many mail options to pick from. If you are sending a letter, the prices are generally reasonable. If you are sending a package, though, prices can get very expensive, very fast. It’s sometimes best to find a mule, aka a coworker, friend, or relative, who will be traveling to the country you’re shipping to who is willing to take the package with them and mail it once they arrive. If you do this, don’t seal the package, in case their bag is searched. And, of course, you cannot use this option if you are shipping something that is not allowed on airlines.
They will give you any paperwork you need to fill out at the counter. You pay. The paperwork you’ll need to fill out and the price you will pay depends on what you are mailing, where and how. If you feel like maybe you are being taken advantage of, just mention that you will need a receipt.
You can buy stamps to have on-hand, if you wish, but you will still need to go to the actual post office to mail the letter, so it really only saves a tiny amount of time.