In the buildings tab of your build menu, you can select the services sub-tab in that. Then you can either select to build a small janitorial building or a big one. Once connected to a path and opened, these people will come out when someone has puked after a ride to clear it up or when someone has dropped litter from shops. The service buildings have limits to how many people of that job you can hire.
You can prevent getting litter on the ground from food places by placing some bins in your park.