Heads up
Earlier this month, a gigantic collection of email addresses and plaintext (meaning unencrypted) passwords was discovered circulating the web, involving over two billion combinations. Known as "Collection #1", this is a huge security risk and very, very easily could have affected everyone here, hence the dedicated announcement.

What to do
First of all, don't panic. These aren't an ongoing problem, they were gathered once and after changing your password on an affected account it will no longer be vulnerable. Use services like haveibeenpwned.com to check your email addresses and passwords to check if they were potentially affected, and which passwords you'll need to change.
https://haveibeenpwned.com/
https://haveibeenpwned.com/Passwords
https://haveibeenpwned.com/PwnedWebsites

Second, get yourself a password manager of some sort. Google has one built right into Chrome, which is actually quite nice, but it doesn't really work for anything besides Chrome, so you can't really use it for game passwords or whatever. A better idea would be to get something like LastPass or 1Password, which are dedicated password managers that, as the name would imply, mean that you only have to remember one password, and it's the last password you'll have to remember.
https://www.lastpass.com/
https://1password.com/

Third, stop reusing old passwords. If your password appears in that list in conjunction with a username you've used it previously on, any future websites you make an account on using that combination are automatically insecure. The point of a password manager is to allow you to have hypercomplex passwords that are all unique to each account ("ceGXe2*1KF&Dxz2hYk6V" is one that I generated just now with LastPass). If you can remember all your passwords, then there's a problem.

Final notes
I know this is work and you probably might not want to do anything about it, but the truth is this kind of thing is actually a huge problem, and could be potentially catastrophic if a password you used for, say, your bank account was one of the ones that were breached. If you don't particularly care about an account on something, then that's fine and you shouldn't worry about it unless, again, it has access to your bank information. Once you get a password manager, though, it'll actually make your life quite a bit easier and a heck of a lot more secure than it used to be, making it one of the rare security measures that actually removes work.