Can you help me
Are you able to help me make up my own security key for my wireless internet
The length of the passphrase depends on the type of security used -- WEP/WPA.
The usual advice is to include a jumble of numbers and letters (capitalised and lower case) but this usually results in something you will be unable to remember without writing it down (which isn't so smart).
A compromise is to use something like a friend's name or deceased pet's name and add some numbers or another word. I suspect it's a good idea to avoid as far as possible using a word which contains the same letter repeated (for example password) -- because as a fan of word puzzles I know that this can be a clue to the rest of the word.
Generally I write user names and passphrases in a book only I can access easily so that when I forget at least I know where to look.