Postcards are for direct mailing and often enjoy a lower postage rate than envelopes. They are more vulnerable to damage and marking, but that, and the stamps and cancellation, may add to the charm.
The stamp goes in the upper right. It must be on the same side as the address. This, and some fluorescent dye on the stamp, is used by the sorting machines to face (orient) the mail piece, so the machine can OCR-read the address, which can succeed if you print neatly. This will greatly speed up processing.
In the US at least, postcards are First Class aka Priority Mail. And will be handled at that speed, except for the handwriting issue.
This is not obvious, but they put their ugly barcode exactly where the stamp must go. This is to not waste space and to help hide the barcode. The local post office may be able to advise which stamp or combination of stamps will fully cover it. Nothing more romantic than a jumble of mixed foreign stamps, you might even do that gratuitously!
To move internationally, certain specific stamps may be required. Extras / overpaying postage is harmless on postcards, but don't do it on packages.
The address goes on the provided lines. If there are two sets of lines, the lower or right one is for the main address.
The return address should be smaller, above, and if possible to the left. The return address is meant to be the sender's, but who would know?
If you hand walk it into a post office, some clerks will cheerfully cancel it "old school" for the classic look.