Lots of learning will be required on your part. Not every game has an editor to edit the games, like Chrono Trigger and Super Mario World do. Some games don't have any information found out about their inner workings at all (data locations, etc..).
I can give you quite a few links, if you are serious about rom hacking, then bookmark all of them, read everything you can, and follow links (and their links, and their links, etc...)
For Chrono Trigger hacking, just look through these boards for programs and hacking stuff. This is the largest (only?) chrono trigger hacking board on the internet. Seriously. If you come across the names of programs, but no link to them, try a google search for that term.
http://www.romhacking.net - A huge fairly new site with a massive collection of documents and tools and editors and a nice forum and and and *explodes*
http://www.romhacking.net/start/ - Romhacking.net's getting started area. Great for new people to the scene to learn some of the basics. Remember, alot of what you learn (if not most) can be carried on and used in different games. If I know how to change the text, graphics and colors of one SNES game, I can use the same tools and techniques in other SNES games.
http://board.acmlm.org/forum.php?id=19 - A large forum for rom hacking, with many backpages of old info for a variety of games and romhacking discussion. Great to read through.
http://board.acmlm.org/archive/forum.php?id=9 - An older version of Acmlm's with even MORE threads about romhacking (the board got wiped a few times, so this is an old archive, do not post in here)
http://acmlm.overclocked.org/board/forum.php?id=9 - An even OLDER version of Acmlm's romhacking board
http://www.zophar.net/wwwthreads/postlist.php?Cat=&Board=romhack - The decaying Zophar message boards romhacking section. Not as busy as Acmlm or Romhacking.net, but it's been around forever and some good stuff can be found out here too.
It would be a good idea to spend a good month or two just reading up stuff in your spare time, and experimenting with roms you are hacking, just to get the hang of it. Don't just jump in to your favorite game right away and attempt a full-blown modification. Start small, change a graphic, change some colors, get used to editing things in HEX, learn hex...
There are probably many more sources of hacking information and tools and utilities, but I don't have the time to post them all, they can all be reached through links off of the above sites and in forum posts.
You can probably learn more from the forums than any document can teach, because you can see other new people like yourself learning, and see them asking similar questions, and feeling their way around and getting the hang of things. Before posting on any forum though, read all the sticky/pinned topics first. Most commonly asked for information and links and rules are posted there, and prevents redundant threads. Some forums are very strict, so make sure to at the very least read the stickies first before asking a question. Rom Hacking forums can be quite fierce when questions are posted, and the answer lies in a sticky, but not all boards are like that.
If you have any more questions after you look around, feel free to ask here, alot of us are still learning too, surely whatever it is you need help with can be answered by one of the many talented people we have hanging around here.
http://www.geigercount.net/crypt/index.html - Temporal Flux resides here, you should download this if you plan on editing Chrono Trigger. Read the readme and any other document that comes with it for information on how to get it to work. ALOT of Temporal Flux usage information, suggestions, tips and threads on this board alone will answer alot of your questions on how to use it.
