This is the first time that my boyfriend and I are going to have dinner together.
This is our first dinner date for just the two of us.
1) This is the first time that my boyfriend and I are going to have dinner together.
「彼と私とでディナーを一緒にするのは初めてです。」
This is the first time that ... で「…は初めてです」という言い方です。
2) This is our first dinner date for just the two of us.
「２人だけで初めてのディナーデートです。」
This is our first dinner date「初めてのディナーデートです」と言っても良いかなと思います。
just the two of us で「二人だけ」
ご参考になれば幸いです！
日本 
