Memorize the hiragana and katakana phonetic systems with something like this quiz tool: http://www.gyford.com/japanese/
Then learn only the basic particles and verb conjugations from: http://www.guidetojapanese.org/grammar_guide.pdf
or somewhere else.
Then decode Yotsuba&!, a children's manga with Furigana, that is the phonetic characters written above each kanji. The basic-level grammar of Japanese you'll find in Yotsuba&! is surprisingly simple, so simple you could read it by having a single index card worth of grammatical rules next to you.
Look up words you don't know at http://jisho.org/
Then you've got your foot in the door.