A lot of phones (of the smart variety) automatically convert the leading 00 to a + when adding a phone number to the contacts list. Whether it's the new 'standard', who knows as long as it works. Keep in mind that while your current state of residence may use 00 as an international access code, there may be other states that do not.
As for Whatsapp, to get it to work you need two things:
- install the Whatsapp app, and go through the 'setup' process. It's been a while but I think you get a text message at some point that confirms the link between the installed app and the phone/number. The app also needs to be able to access the contacts on your phone. If you've successfully used Whatsapp before on this phone, this is most likely already sorted.
- Make sure that the persons that you would like to contact via Whatsapp are stored in the contacts list on your phone. As for how to format their numbers... you'd best check that in the phone's documentation (I know, that doesn't exist anymore, but Googling 'storing international numbers in <model phone>' should do the trick).
If you've taken these steps, you should be able to initiate a new Chat in Whatsapp, just tap the relevant icon (top right) and select the contact from the list. If the person doesn't show up in that list, there may be something wrong with the way you've entered their number, or they do not have Whatsapp installed correctly on their end.
Hope this helps.