There are plenty of tools across the internet that will help you generate random passwords. I recommend using a password manager, and many password managers come with a password generator built in. I've recommended one above in the "Tools" section of this guide. These are generally reliable for creating secure passwords. As a guide, an 8 character random password usually takes a few days to crack whereas a 10 character password takes years. Obviously the more characters you use the more secure the password assuming it is completely random.
Once you generate your password you can go to howsecureismypassword.net. Simply enter your password, and it will let you know how long it takes a normal desktop computer to crack it!