Sally is an American who lives in Japan and speaks Japanese. Billy is a small boys name Bill prefers to be called Bill as he is a man. Also Biru is the name the teacher assigned in the presentation outline.
Just focus on the particles and conjugation.
Bill- Hai Amerika jin desu. Watashi wa nihongo ga wakarimasu. Watashi wa GCC* no daigaku de benkyō o shimashita.
(はい、アメリカ人です。私は日本語が分かります。私はGCCで勉強しました。)
Sally- Sō desu ka. Doko ni hatarakimasu ka? ( or doko ni shigoto o shimasuka? or shigoto wa doko desu ka?)
(そうですか。どこで働いていますか?(or どこで仕事をしますか? or 仕事は何ですか?))(second one is not good for this. I think the last one is best.)
Bill- Watashi wa disney ni hatarakimasu. Watashi wa shutchō de nihon ni kimasu ( or kimashita im not sure on tense ..i came to japan for business trip or i come to japan for business trip)
(私はディズニーで働いています。私は出張で日本に来ました(来ています)。(I have a question. In this situation, which is correct in English, I come to Japan or I came to Japan? ))
Sally- A sumimasen (sally calls for bill) Biru san wa ashita doko no ikimasu ka?
(あ、すみません。ビルさんは明日どこに行きますか?)
Bill- Ja.... Watashi wa ashita no asa kaigi ikimasu
(えーと、私は明日の朝、会議に行きます。)
Sally- Ja.... ashita no ban wa watashi no pati desu. Biru san ni ikimasu ka?
(明日の晩は私のパーティです。ビルさん、行きますか? But this one sounds strange and desn't make sense though.)(ex: 明日の夜、パーティを開く予定です。ビルさん、来ませんか?)
Bill- Sō desu ka. Anata no pati wa nanji kara desu ka?
(そうですか。あなたのパーティは何時からですか?)
Sally- gogo hachi ji han made desu.
(午後8時までです。 This one isn't correct. You should say 8時からです。)
Bill- Ja.. watashi wa Hirari san to, Cherushi san(or chan or kun i have no idea of honorific to use as it is a father referring to his daughter to a stranger.)to ni ikimasu. Daijōbudesu ka?
(平井さん、チェルシーと一緒に行きます。大丈夫ですか?)You don't need to use san, chan or kun for your children in this situation.
Sally- (dismayed) Hai.... daijōbudesu... ja shitsureshimasu.....(goes back to tend store)
(はい大丈夫です……orz。では失礼します……。)
I think one of the usage of が is similar to a/the in English.
There is a magazine on the table. The magazine is the latest issue of Koro Koro Comic.
(机の上に雑誌が乗っています。その雑誌はコロコロコミックの最新号です。)
I'm sure you don't have a problem with this usage though