How to create a local database using JSON and JavaScript for webapp

I need to develop a simple webapp with a login system, users and data saved to each one of those users, with no knowlege of PHP or Data bases, only HTML, JavaScript/JQuery and JSON.

I must be able to sign up, log in and save some data (it's an online store, i must be able to save purchases historic, list of avourite products, user preferences, etc), all this locally only.

What is the best way to do this?

Security issues are not relevant.

I'm sorry for the bad english, I'm not a native speaker.

Hope you can help me!