Ideally, a password should be easy to remember, tough for someone else to guess, and complex enough so that computers cannot easily crack it. Any one of these three is easy, but having a password that satisfies all 3 is harder.
Easy to Remember
It's much easier to remember one password then 20 passwords, so try to choose a password that can be used on different computer systems and on many websites. This usually means a password with at least 8 letters (and sometimes no more than 8). It should also be complex, as many websites have complexity requirements. It may be best to chose two or three passwords so that a short one can be used when security is not a concern and another one or two can be used where security is more important.
Tough to guess
Do not use the name of your cat, your birthdate, the word 'secret', or any word in the dictionary of any language, or any password that anyone else in the world may have used before.
A good password will have a mix of uppercase letters, lowercase letters and numbers (you can also have symbols, such as * and #, though many websites will not allow this, so try to avoid these).