Hi sortof,
It seems that you are really enthusiastic about learning Japanese. Good Luck.
I have to agree with everyone else that the first step you need to take is to learn Hiragana. Try as much as you can to avoid writing anything in roumaji. Hiragana is a set of phonetic characters used to write in Japanese.
Watashi would be written in hiragana as わたし, although in Japanese it will actually be written as 私 (in Kanji), although the hiragana may be written on the side for spelling. わたし is still proper japanese though, and its what you should write yourself.
Roumaji is not used by Japanese and not necessarily phonetic as there are many systems using hiragana as soon as possible will save you a lot of worry. Have you worked out how to use the Japanese IME in your language bar on your computer? To learn hiragana just practice writing out words by hand, looking up the kana when you forget them.
I also agree with absolutely everything Chabichou says, although I didn't use Genki myself.