Reading the scripture in common worship Do all Christian Churches/Denominations have a common set of readings they read throughout the year and repeat on a 2-3 year cycle? 

If not, how do pastors in churches without a common set of readings decide what they're going to talk about in any given week?
No, certainly not. There are lots of available resources. Some denominations have guidelines and some reference materials exist for those who do not (such as the Revised Common Lectionary), but by no means do all traditions have set or repeated liturgies.

When it falls to me, I build each weeks liturgy based on passages that have some bearing on the sermon topic. Often these are passages that have a history of being used in liturgies, but that isn't a requirement. Each week takes time in research and prayer to decide what these will be. As church elders we settled on a general format of for the liturgy and the kind of readings to include, but each week varies some to accommodate the main passage. The sermon material follows an exegetical path through whatever book we happen to be working through. We do use fixed readings for confessions and Lord Supper, but improvise to work those readings into the overall flow of the service.