You should look for LCAs on biogas production. For each LCA you find, you will be able to see the system boundaries that the authors used and which impact assessment method they used. This will give you an idea of what is typical in biogas LCA studies. When determining you system boundary, you first define the goal of your study, and figure out if you need a cradle-to-gate or a cradle-to-grave study. Then you can draw up your system boundary for your study, starting with raw materials.