I'm not just going out to have fun tomorrow. I have to go to city hall to get a form for my pension so I can pay it.
この場合、次のような言い方ができますよ。
ーI'm not just going out to have fun tomorrow. I have to go to city hall to get a form for my pension so I can pay it.
「明日は遊びで出かけるんじゃないよ。市役所に年金を払うのに、払込書をもらいに行かないといけないんだよ」
a form for one's pension を使って「年金の払込書」を言えます。
ご参考まで！
カナダ 
