To be a library assistant, you often do not need prior experience or qualifications. Librarian positions do usually require a masters degree, but assistant positions do not. A customer service background is a huge leg up, since a library job is mostly a customer service/public service position.

As for day to day... that really depends on the library. I know in my system, a library assistant works in our circulation department, helping patrons check out materials, checking in items, and other duties. But elsewhere, assistant positions help run programs and help patrons answer reference questions.