How do I prevent emails from going into my 'Junk' folder?

It depends on where the junk filtering is occurring. Email is typically analyzed for junk and routed to the Junk folder on the server. Therefore, the settings for Junk processing depend on your email provider (i.e. Apple iCloud, Microsoft, Google, Yahoo, etc.) and the settings usually need to be controlled there by logging into your account via a web browser.

As far is iOS junk filtering goes, make sure that you have any expected senders in your contacts, and, if an email is marked as junk incorrectly, then mark it as "not junk" using the procedure in the document: Report and reduce spam in iCloud Mail - Apple Support