for my project I need to display a landscape's susceptibility to erosion. I created a slope-map out of my DEM and added the bedrock geology as a polygon-feature shapefile. Now I want to tell ArcGIS that areas with a certain slope (lets say >30°) and a soft rock underlying (e.g. schists) are particularly affected by erosion, whereas plain areas with harder lithologies are not. I used the Raster Calculator tool before to produce hazard maps, but I never used a shapefile as input. How can I transform the information stored in my shapefile into a raster?
You should try the Feature to Raster Conversion tool.
The conversion relies on an attribute which will become the "numeric" class of a raster, if you would share the attribute of your shapefile we might be better informed to guide you into which options you should pick for you conversion (but in general, I would imagine that to be self evident).