I think i'll start trying to learn Japanese first. It might be harder, but it will be easier to find a pen pal on the internet, possibly even on these forums.
I speak a little japanese. I've been to Japan. And I had a japanese pen pal. Couple comments and general suggestions:
1) Even after one year of college courses, hundreds of hours of watching anime, half a Pimselur course and an instant immersion computer learning CD...even random people walking the streets in Tokyo spoke more english than I spoke japanese. Learning to speak japanese well enough to be useful at all might take years.
2) It's much easier to find penpals who want to write in english than it is to find penpals who want to write in japanese. They also can't generally help you with pronunciation. Also, writing in japanese is a non-trivial hurdle above and beyond speaking it.
3) Learn hiragana before you learn katakana. Incidentally, if you don't know what these are, japanese has three character sets: hirgana, katakana, and kanji. Hiragana and katakana each have either 48 or 104 characters...depending on how you count them. These two character sets are completely redundant. か is ka in hiragana, and カ is ka in katakana. Which one you use depends on the word. Except when it doesn't, and people just use the wrong one to draw attention to a word, sort of like italics. And while ka looks pretty similar for both, not all do. For example, か and カ are both ka, but た and タ are both ta. Incidentally, be aware that in addition to this...if you want to be able to read at even the 6th grade level, there are another 1006 kanji characters for you to learn, and roughly 2000 to be able to read at an "adult" level.
4) There are some nasty pitfalls when it comes to reading that your life will be much easier if you know about in advance rather than trying to figure them out on your own. For example, o (お) will generally be substituted with u (う) in the case of a double vowel. Which means that you might see a word, look it up...and it doesn't exist in the dictionary because the romaniztion rules and kana rules don't completely agree with each other. つ is pronounced tsu, but when it's
smaller it's also used to indicate double consonants. For example, while あさり is pronounced asari, あっさり is NOT pronounced atsusari, it's pronnounce assari, with a tiny pause between the two s's. お and を are both "o" but they have completely different use and are not interchangable. And sometimes, some people, will pronounce を as wo instead of o, and that's completely correct and valid. Be careful about this kind of thing if you're learning on your own.
Also, to the guy saying Pimsleur courses are worthless...sorry, I disagree. Even after an entire semester language course, even the basic Pimsleur 1 course will still be valuable. The Pimsleur method might be
insufficient on its own, and I acknowledge that. It is a very deep tool, but it is not at all broad. If you do a 30-day pimsleur course in the manner they recommend it, at the end of the 30 days you'll only know about 500 words. That's not very many. But...you'll be able to use those words completely comfortably, at conversational speeds, with good pronunciation, and you'll be able to create sentences from those words on the fly. Compared to tools like rosetta stone or instant immersion, which will help you memorize countless individual words...which you'll be completely unable to use or recognize them when actually spoken in a sentence. I highly recommend pimsleur. Just don't expect that method alone to be enough to allow you to become fluent. That's not what it's for. In a classroom environment with 1 teacher and 20 students, very little time will be spent engaging is speaking
practice with a competent speaker. Sure, you might partner with with other students, but the students you're partnering with generally don't know what they're doing either. A typical 30 minute pimsleur lesson is 5 minutes of instruction combined with 25 minutes of nasty, hardcore drilling of listening to speech and responding to speech. It's a very good tool, when used for the specific purpose it is intended. A saw is good for cutting wood, but it's silly to expect to use a saw alone to build a house. Pimsleur is a heavy duty power saw...but you're going to have a difficult time building a house with it unless you have the hammer and nails too.