This answer might shock you, but the truth is, you cannot learn piano online without the guidance of an instructor.
Sure, you will be able to get through some basics and some fundamental playing skills, but past that, you will need an online program that includes a teacher who will be available to you in a variety of ways.
The most important part of searching for piano lessons online, or any learning system online is to make sure the support system is the core part of the learning system.
I repeat, the support system is the very heart of the learning system. The support system is NOT just being able to send an email and get it answered, although you need to make sure you have that option… you should also make sure you have a variety of other support options such as live broadcasts, thorough FAQ, lots of blog entries, video instruction, etc.
It is very important that you feel like you have a good relationship with your online piano teacher.