The First Thing to Learn

I've been thinking about getting back into the Pokemon ROM hacking scene, and I've got a base idea on what I want to do, but what is it that I need to learn first when it comes to ROM hacking?

Get a hex editor, open up a FireRed ROM, scroll down and look at all those FF's(Free, unused space).
There's a lot of free space in a Fire Red ROM, kids. That's the first thing you should know.

I don't really know too much about ROM Hacking but I'd say its pretty important to have a story of sorts in mind and be open to changing bits and pieces to make it fit your end goal.

Not to mention being open to reaching out for help on things you don't know how to do but I'm not too worried about that with you based on this thread showing you can ask for help.

You can do everything in hex, it's the most important thing to learn and be comfortable with. Any time something corrupts for me, I go into a hex editor, see exactly what is different, and fix it all manually. I never have to wonder what went wrong.

But I mean, if you actually mean the first step in the process, I'd say get Advance Map, and mess around with changing things in the world. Also, the tutorial built into XSE is genuinely good, and includes a lesson on hex.

Not so much a skill but just a good thing to keep in mind ALWAYS.

ALWAYS ALWAYS ALWAYS BACK UP EVERYTHING!

This doesn't just mean the ROM. I'm talking about literally EVERYTHING.

Make a script? Put it in Notepad or something and save it somewhere! I can't tell you how many times I've had a map blow up or a script corrupt or simply have a script be out of date because I wrote it three years ago. I was always so glad to just bring it up in the script editor through a text file and not have to rewrite it over again.

Make a map? Save a image of it through AdvanceMap. I recommend using the Event View when you do this too. I've had to redo quite few maps because something or other corrupted (again).

On that same vein I also screenshot both the Map Header and the Professional Map Header (press ctrl+H while in Header view). The second one can be very useful if the map footer of a map gets overwritten or messes up. If you don't have a map footer, your map is basically useless, so it is very important.

Even if you're smart and play it safe, things can still get corrupted for seemingly no reason, especially if you're testing something for the first time.

Another tip (besides getting familiar with how to hex, because other people have said that already) would be to look at the .ini files for tools like PGE or G3HS and see where all the tables and such are. Learning how those work, how long they are, their format etc. can help if you need to edit something and a tool doesn't work or you need to restore something.