Just enter your phone number (7 to 10 digits, with or without dashes) into the box on the home page and click the submit button.
PhoneSpell can help you pick a new 7-digit phone number. In North America, the first 3 digits of a 7-digit phone number specify an "exchange." When you are ready to order a phone, the phone company can give you a (typically very short) list of possible exchanges (3-digit prefixes) your new phone number might be able to have. If you are getting a phone number within a large institution, their telecommunications department might tell you that your phone number must begin with a specific 4 or even 5 digits.
If you type 3 to 5 digits into the box on the home page (and click the submit button) PhoneSpell will give you a list of all the 7-letter and 8-letter words that correspond to phone numbers that start with the digits you entered. Any one of these phone numbers would fit the restrictions of the phone company and would have a one-word mnemonic. The only thing left for you to find out is if that phone number is available (i.e. not already in use or reserved for some reason.)
In addition, PhoneSpell will list all the words found within the digits you entered. Suppose you entered "843": PhoneSpell would tell you that this spells "the" (what luck!). Knowing this, you can make your own 2-word mnemonics: simply pick any 4-letter word that you would want to follow "the" and convert it back to digits (see the next question).
Lazy, aren't you? Type the word into the box on the home page (and click the submit button) and PhoneSpell will translate them back to digits for you. You can type in a mix of letters and numbers (e.g. 555-BEST) and PhoneSpell will leave the numbers alone as it translates the leters (in this example, giving you 555-2378).