YOU SAID:

i have to go get some at the party this weekend

INTO JAPANESE

私は今週末のパーティーで何かを買いに行かなければなりません

BACK INTO ENGLISH

I have to buy something for the party this weekend.

INTO JAPANESE

今週末のパーティーのために何か買わなければなりません。

BACK INTO ENGLISH

I have to buy something for the party this weekend.