You need to first pick a password manager, imo bitwarden is the best. You set up which email and master password, and then you start going around to all the sites you frequently use and logging in, so the password manager offers to save the password. Once you've got your main sites, usually there is a security option that shows you which sites have weak passwords or where you've reused a password a lot (on bitwarden it's only on the site, not the app https://bitwarden.com/help/reports/). Those are the ones that probably need changed first. You can then add your less frequently used sites over time as you use them.
It can be some work to initially set up, but once it's done you're pretty much set. I went from reusing the same 3-4 passwords everywhere to having a unique login for every site. Seeing 210 saved logins really puts into perspective how much a password manager helps tbh.
No, just install it and every time you use a password that isn't in the database you'll get prompted to add it. One of them major benefits though is being able to use large, complex, random passwords. So, it is strongly recommended to switch your passwords over. You can do it as fast or slow as makes sense for you.