Popular posts is another matter; you'll need an add-on to track visits, then get the ones with the most visits. Or use a star-rating system, or some other way of determining "popularity".
For a simple theme originally designed for blogging but which would easily be customizable for your purposes, check the Flatso package. Again, you can find it through the package manager.
Be aware, it will completely overwrite your MODX installation with its own templates, extras, and resources. It is a full blogging site package. https://github.com/modxcms/flatso