For 1: I think the best way to start is to simply schedule your important TODOs (C-c C-s), so that you see them on the weekly agenda view. This is how I started, and then I slowly started to incorporate things like tags when I realized I needed some more organization. (And make sure to add your org files to `org-agenda-files`.)
That's a mostly up-to-date list of supported languages, with links to example usage.
Technically, org-babel will support any language, but some have more interoperability with the overall system than others. At the simplest, you'll get syntax highlighting, tangle, and noweb support. At the most complex, you can execute them inside emacs/org and get the results, feed those into more blocks, etc. (making it very much like a python notebook or similar concept, but for any supported language).
You can even see the org file generating each of those pages.
1. org agenda integration into their todo lists 2. babel mode? i'd love to integrate code into some of study guides for interviews
love emacs, but i never feel like im using more than 1% of org mode's potential.