https://bitwarden.com/

This is your new friend.

It'll generate very complex passwords for you. You just need to copy and paste them. The software is open source (which means many eyes are on the code and you can trust it, you can even compile it yourself). You can access your password vault from all devices.

To start:

The most important first step is to choose that master password. I would recommend choosing something random which looks easy to type and writing it down on a piece of paper. Don't try to memorise it straightaway, focus on entering it exactly as written on the piece of paper. Once you've entered it a number of times, try putting it out of sight. If you're still able to enter it then move it to a safe or similar.

The next most important thing is to set-up two step login for your BitWarden account. If you're using BitWarden Premium then the most secure is a YubiKey. If you don't want to invest in one yet then an authenticator app is a good option. Print your BitWarden two step login recovery code and put it with your master password.

Next secure your email account. (I recommend protonmail) Choose a strong random password, save it in BitWarden and enable two factor authentication. To minimise the risk of lockout, its a good idea to print your email login credentials and store them with your master password.

Then over the coming days and weeks, go around all the websites you use and change the passwords to something unique/random, save them in BitWarden and enable two factor authentication where available. Start with the more critical ones, like banks, email, cloud storage, etc. For the others you may want to wait until you need to visit that website or receive an email from them. The important thing is that you get around them all in the next month or two.