Speaking of vague... I'll assume you're talking about learning Japanese.
Anyway, the easiest way to learn is to take advantage of your school if they have a Japanese language program.
Edit: After reading the post below, bungiefan brings up a good point: You need to practice with other people in a variety of situations. That's where the classroom comes in, which is another reason why taking a Japanese course at school is the easiest way.
Otherwise, try out any books you can and see which one works for you. There are free alternatives, but you can only learn so much from freebies. If you don't really aim to be fluent but still want some level of comprehension, the least you can do is learn katakana. If you don't know what that is, look up a little more about Japanese language. Katakana is used mostly for foreign words, mostly English when it comes to games especially Tales. So if you understand katakana, you'll be able to pick out a lot of English, so menus will be mostly easy to navigate. You'll eventually figure out which menu option does what.
Anyway, after you learn katakana, learn hiragana if you have any effort left. Katakana is significantly easier to learn, hiragana is a bit more difficult but you should learn if you keep at it. Then try looking up commonly used kanji. It may do you some good to learn to write all these if you were planning otherwise. Writing them burns them into your mind faster.
Absolute minimum hiragana you should know:
はい = yes
いいえ = no
http://aselia.cjb.net/Cyllya's site above has some good advice on learning Japanese. Enter the site and click Japanese on the sidebar. She has charts for katakana, hiragana, and a bit of common kanji used in RPGs such as elements, towns, etc. I myself got started with learning Japanese by going through Cyllya's kana charts, then started taking Japanese at school.
For Tales of Rebirth specifically, Spekio's translation guide below does almost exactly what you're asking. It translates the text boxes throughout the game, most likely just the main story and not much sidequests so you're out of luck there.
http://www.gamefaqs.com/console/ps2/game/920236.htm