A password ?
You use passwords any time you need to login into some account: forum, email, bank... It is the very first protection against unauthorized users.
However, passwords are always subject to one type of attack: brute force. This means that the attacker try to find your password by trying each possible password: so the complexer the password, the better.
With a 8 chars password, the number of possible passwords are:
Only numbers: 100,000,000
Only tiny letters (a to z): 208,827,064,576
Capital and tiny letters: 53,459,728,531,456
Capital and tiny letters and numbers: 218,340,105,584,896
All symbols on a normal keyboard: > 10,000,000,000,000,000
How to remember passwords
The best is of course not to physically store it (write). I suggest then to make a sentence you won't forget (as a proverb, a song) and take, for instance, the first and the last letters of each word, once tiny, once capital.
Example: It is raining cat and dogs
You can then change "s" to "$", or whatever letters you want: