ASCII complies to the ASCII character list. Basically it's alpha-numeric.
I've always just used hex. If you don't want to create a HEX set then you can go to a site that provides a HEX set for a passphrase that you make up. For example (the passphrase conversion utility may also exist in your router), you can enter in the passphrase of your choice "My router has weak security" and click generate and the app will generate a HEX key based on the passphrase. Make it something random. Either way, to be perfectly honest....WEP is weak. very weak. someone with the right tools can break a WEP key in a HOME network in a matter of days. But, no matter. Most people wouldn't even bother to waste that amount of time on a home network anyway. If you have WAP encryption, use that instead. If not......then use WEP and in conjunction with WEP, use a MAC ACL (MAC Address Access Control List). You will have to then enter in the key and also have a wireless network adapter with a MAC address that is included in the list in order to gain access. And you would be the sole person in charge of that list of MAC addresses.