I don’t know about Heilbronn specifically (I live in Heidelberg), but Baden-Wurttemberg is a predominantly Catholic area. You should be able to find a Catholic church with no problem, probably within walking distance of where your host family lives.
Most Germans who attend mass have their own missals, but check in the back of the church as there are occasionally some extras lying about. You will need a missal to follow along completely, as it is a missal/songbook combined. The numbers for the songs are generally projected discreetly on the wall.