I'm quite new to QGIS.
What data will I need to use the
r.sim.sediment processing tool?
r.sim.sediment is a tool from the GRASS toolbox. You can find documentation for GRASS tools by clicking the "Help" link in the QGIS tool dialog for that tool, or go straight here:
It shows that you need a few raster data sets over your study area, including elevation, water depth and so on.
If that documentation is insufficient then chase the references given and/or search for a tutorial or example that uses it - this might be a tutorial based on GRASS rather than a QGIS one but will use the same functions that you can get from the GRASS toolbox in QGIS.