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.

First step is just installing one and using it. Let it save your accounts and current passwords as you use them.

As you add accounts, if you know you reuse the same password for some accounts, change those passwords to a randomly generated one with the manager.

As others have mentioned Bitwarden integrates really well on desktop and Android. Highly recommend it.

I just switched recently from using insecure passwords to using KeePass. I was dreading the change, but it's very easy.

I changed all my important passwords to highly secure, randomly generated ones and saved them to my KeePass database. That's it. Took me like 30 minutes. Frankly I didn't bother with accounts that don't have any important info on them.

The upside and downside of KeePass is you have to keep track of your database file - it isn't stored on the cloud, so unlikely to get hacked, but if you lose the file or your master password then it's gone forever. If the cloud hosting isn't an issue for you and you like that level of convenience, then maybe Bitwarden is a better choice.

You don't need to, but it probably wouldn't be a bad idea unless you were already using fairly complex passwords.

For a lot of them, you set up the service, then as you log into things, it asks if you'd like to save the login credentials you just used.

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.