Keith, Romaji won't cut it for very long if you're serious about learning Japanese

You need to learn Hiragana and Katakana post-haste (like yesterday).  Learning a few basic Kanji will also help.

However, this is a good start. Keep up the good work

私 = kanji for watashi

は=wa

です = de su [で=de す=su]

watashi wa keith desu.

私はキースです [キース(katakana) = Kīsu]

watashi wa amerikajin desu. [little kids say "boku", adults say "watashi"; lose the "mo"; it means "also" as in "my wife is American; I am American, too"]

私はアメリカ人です [アメリカ (katakana) = a me ri ka ; 人(kanji)= jin]

Watashi wa juukyuu-saidesu

私は19歳です [歳 (kanji) = sai]

shumi wa chesu desu.

趣味はチェスです [趣味(kanji) = shumi; チェス(katakana)= chesu]

e o kakukoto mo suki desu..

絵を描く事も好きです [絵(kanji)= e;描く事=kaku koto;も(hiragana)=mo; 好き=suki]

watashi wa nihonjin dewanai.

私は日本人ではない [日本=nihon 人=jin; ではない=de wa na i]

ありません=a ri ma se n