You don't need library training to get hired at the circulation desk, but a few specific skills put you ahead of other applicants fast. Learn call number order and cash handling before you apply. The system work and the supervisor skills come once you're inside.
Worked the front desk and helped customers with their accounts.
Processed 300+ checkouts and returns per shift in Polaris, resolved fine disputes and balanced a cash drawer with zero variances over 12 months.
Learn call number order. It's the one library skill that interviews test directly, and you can practice it at your own branch by reading spine labels. Cash handling and customer service you likely already have, so put those front and center on your résumé.
Not a widely required one. Libraries hire for this role on experience and a skills check, not credentials. A few states run support staff certification through the state library, which can help later if you want to move up.
You can't pick in advance, since each library uses its own. Look at the catalog for the system you're applying to, since the vendor name often appears at the bottom of the page. Knowing the concepts, like holds, transits and blocks, matters more than any one product.
Learn the reports, volunteer to train new clerks and become the person who knows the policy manual. Supervisors are usually promoted from the desk, so being reliable on the hard parts of the job counts more than seniority alone.