Well if you're not willing to spend a whole bunch of money start out with something simple. My Japanese Coach(DS) may be a horrible attempt at teaching Japanese, but if you're not one to spend money, or you're too uncomfortable to take classes or you're to young to, it's a start. To be honest the only thing I actually thought was useful in the game was the dictionary of phrases. There's also another Japanese learning aid coming out next week for the DS.
If or when you get to the point where you have a good vocabulary, you can go on to the Japanese learning games for toddlers/kids/teenagers.
But seeing as you need to get a flashcart (or spend hundreds getting all the games) and there aren't many PSP apps to my knowledge, you can download some mp3s(pimsleurs Japanese) to your mp3player, if you like the sample they give you, you can buy it at a later time.
This is the only thing I can actually recommend if you're low on cash or don't have internet access all that often.
Though I do recommend getting a mic or headset when you do get your internet back, it helps to get some help via Skype or other V.Chat programs, who actually offer Japanese in return for English lessons. Eventually you're gonna have to either get someone to talk or write to, or take classes(my community college is $150 for 5months of classes)