Do people decide on the books they want to work on or is it just given or random?
It depends. Unless you're a translator or can get a translator to work on the project, you won't be able to start your own project. You can always try to apply to help out with a series of your liking, but it doesn't mean you'll get accepted - popular series usually are staffed quite well.
Manga Editor websites should have a recruiting subpage, where they'll usually post series they are looking staff for. So if a position is open, you can apply to try your luck.
Of course, you'll probably have to pass a test to see if you're qualified enough.
Cleaning takes a lot of work and depending on the series, typesetting might too. Plus, you're likely to be asked to save cleans as .PSDs, same with typeset files - they're gonna take quite a bit of space. How much can you upload at your school/library?
Also, since you probably won't be online more than once a day, you're not likely o be asked to work on any popular projects - waiting at least 24hrs for an update from your teammates is rather annoying.
Anyway, proofreading is the easiest of the bunch, so if you're not familiar with Photoshop and can't go online at home to check stuff out, it should be what you should focus on (proofreading).
Still, not being able to look up words, random facts, etc, is gonna make your work harder.
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I'll ask for the thread to be moved here:
https://mangahelpers.com/forum/forums/staff-and-project-resources.860/
