Online or desktop? If you want to learn QB Online, you can sign up as a proadvisor for free and you would get a free account that you can use to practice on. They also have a lot of training materials online once you sign up.
Desktop is pricier (as in, not free). I use both extensively.
A QuickBooks ProAdvisor is someone who is in the QB ProAdvisor program. For the online version of QuickBooks, QBO, there is no cost to join. Once you join, you can become certified in QB Online by using their training, and you get a QBO file for your own business. You will also be listed on their Find A ProAdvisor website, which can be useful or not, depending on how many other ProAdvisors are in your area. However, with the online version I have no need to be located where my client is, so finding someone with a few miles isn't an issue for my clients.
For QuickBooks Desktop, there's an annual fee, somewhere in the neighborhood of $600, and that doesn't include payroll which is another 300 annually. (Same for QB Online - payroll is extra from the subscription fee,) Once you're in the program, you get the annual release of the new version, and you can certify in QB Desktop by studying their training materials and taking the test, which is all online.