An easy place to start is to try making a table for a game, then find some text in the rom (using a hex editor) and change it. Some games are easier than others, so make notes as you go and don't be afraid to switch games if you get stuck...you can always revisit that project later, once you've learned a bit more (hence why I recommend taking notes)
Some tools I use are YY-CHR (graphics), windhex (hex editor), cartographer (script dumping), atlas (script insertion), notepad++ (text editor), and mednafen (the best emulator). There are plenty of other tools out there, so find whatever works for you, or even make your own
and browse romhacking.net -- there's a lot of good information there