YOU SAID:

I just joined the party of Brett but Amanda said that this party is boring and she wants cake

INTO JAPANESE

ブレットのパーティーに参加したところですが、アマンダはこのパーティーは退屈だからケーキが欲しいと言っています

BACK INTO ENGLISH

You just attended Brett's party and Amanda says she wants cake because the party is boring.

INTO JAPANESE

あなたはブレットのパーティーに出席したところですが、アマンダはパーティーが退屈なのでケーキが欲しいと言います。

BACK INTO ENGLISH

You just attended Brett's party and Amanda says she wants cake because the party is boring.