The typical answer is get PND Manager onto your Pandora as soon as possible, you can then sync that with the repository and it'll list almost all software available for the Pandora. You can browse the 'top' category (for highest rated), the newest releases plus search titles, categories etc. It doesn't answer all your questions, but will certainly get you started with a lot of cool stuff!
Next I'd probably have a look at pandoralive.info it has a few articles that list a bunch of favourite software (from the authors point of view), might not be your cup of tea, but I'd recommend grabbing it all to see! This article has a bunch of GL games
http://pandoralive.info/?p=1533 and he also has weekly software round up you can easily find.
If you then browse the software section of this forum you'll typically see on the first 2/3 pages a bunch of threads with lots of posts for software releases, with instructions how to run, so grab all of them!
For ROMs, Google really is your friend, it is very rare not to be able to download a ROM by checking the first couple of search results (obviously ensure you are doing things legally yada yada).
EDIT: thatgui before me has the bases covered, but I started this post before this post was sent, excuse the duplicate information!