On the QGIS wiki there is a great section with the self-explanatory title ‘How do I do that in QGIS‘. To quote from the introduction of this section, “it is intended to show how to perform basic GIS operations in QGIS in the most straightforward way… It includes common operations which are typically one step in a longer process of analysis”.
For more in-depth tutorials, go to the User’s corner with tutorials on the same QGIS wiki. The wiki also contains a page on ‘Ecological tools‘. It basically is a list with suggested steps to come to an ecological toolbox in QGIS and links to existing tools for ecological analysis. Not extremely useful right now, but you might want to keep an eye on it.
There are more tutorials and other sources of information on the use of QGIS, but above-mentioned links are a good start.