There are lots of password managers out there like LastPass, 1Password etc. They are very useful and convenient, but downside is that your sensitive info is stored in the cloud (on third party servers). Data is of course encrypted and stored in a secure way, but there was a number of recorded vulnerabilities and security breaches of these services.
There is a secure and free alternative – just store your passwords in encrypted file on your devices. You can put that file in Dropbox (or Google Drive, Microsoft OneDrive, etc…) folder and keep passwords synced on all your devices.
KeePass is open source password manager – software which manages such database. Passwords are stored in encrypted file and they can be accessed only by master password. So all you actually have to remember is master password of KeePass database and you can obtain all other passwords from that database. KeePass is Windows software, but there are ports for Linux, MacOS, Android and iOS.
I have Ubuntu 18.04 (Bionic) on my desktop and laptop at the moment (trying to move away from Windows ecosystem). KeePassXC is Linux port of KeePass and it works great on Ubuntu. MiniKeePass is excellent iOS port, so I use that on my iPhone. Dropbox app is installed on my computers and smartphone, so database file is synced between devices.
Software is easy to use – first you have to create new database and set its master password. Then you can add entries. You can group entries in folders to keep things organized. Specific entries can be easily located using search box. Each entry can contain title, username, password, URL, notes and additional custom attributes.
If you prefer to use KeePass browser addon (extension) for password manager, there’s number of options for both Chrome and Firefox. In this case you should enable KeePass browser integration (for KeePassXC: Tools -> Settings -> Browser Integration). I prefer to use browser’s default password manager, so I’ve skipped this step.