Right now, we're going to change a word in a woman's speech in the first town in Zelda 2. You won't understand why we're doing what we're doing yet, and that's fine. I want you to get your feet wet right away without burying you under a bunch of complicated ideas.
Open your Zelda 2 rom in Hex Workshop. Go to address E777 ("Edit" menu, "Goto..."). Make sure your editor is set to overwrite, not insert. Type ED EB E2 DB DB E5 DE. Each of those is one byte, a value between 0 and 255. In this case, each of them represents one letter in the text of the game.
(Note that everything we work with here is in hexadecimal (base 16: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, 10, 11...) instead of ordinary decimal (base 10). If hexadecimal numbers are new to you, you may want to go study the basics first. Just know that with two hex digits (one byte), you can represent any value from 0 - 255 (0 - FF). Remember that 10 hex = 16 decimal, 20h = 32, 30h = 48, etc.)
Now save the rom and go play it. Go into the first town and talk to the old woman.
With a few seconds of work, you've changed Link's quest from something involving crystals into a classic episode of Star Trek. Right now, you probably have a lot of questions. How do you know what address to go to? How do you know what number represents each letter? When do we have lunch? Well, have no fear, because all of these questions will be answered in turn.