ーI'd like to pick up all the garbage at the park I've been going to since I was a kid.
「子供の頃からずっと行っている公園を掃除したい」
the park I've been going to since I was a kid で「子供の頃からずっと行っている公園」
to pick up で「拾う」
garbage で「ゴミ」
ご参考まで！
It's a park I've been going ever since I was a child.
I would like to go to the park I've been going ever since I was a child, so I can clean it and make it beautiful.
The park I've been going ever since I was a child is dirty, so I want to go there to clean it.
ご質問ありがとうございます。
「子供のころからずっと行っている公園」はそのまま英語で言いたいなら「It's a park I've been going ever since I was a child.」と言えます。
また、「きれいに掃除したい」を追加したい場合、「I would like to go to the park I've been going ever since I was a child, so I can clean it and make it beautiful.」になります。
また別の自然な言い方ですが、「The park I've been going ever since I was a child is dirty, so I want to go there to clean it.」と言えます。
ご参考になれば幸いです。
カナダ 
アメリカ合衆国 
