The best thing to do first is to positively identify the model of your machine. Google "singer treadle sewing machine" and click on "images" to identify your machine from photographs. Once you know your model, you should be able to Google for a sewing machine manual. There are lots of these vintage manuals available for download, about $15, or you can check eBay for a manual. Often you can also find threading diagrams online by Googling. You really need to know the model number, though.
It's pretty easy to find replacement belts online too. Here is a link to one:
http://www.sewingpartsonline.com/alp...adle-belt.aspx