Probably the reason there is no tutorial for "creating a database" is because it is just one or two lines. Here is a unit test written in Objective-C that creates a document. Once you have that database reference you can use it to create a replication as in the tutorial above.
Who will create the JSON file?
What JSON file? There are no JSON files, just JSON objects. The user creates them.
Where will it be store?
This is not a user-level concern. The only thing you need to know unless you are doing some low level debugging is that they are stored "in the CBLDatabase"
Is there any specific JSON format...?
What does this mean? JSON is JSON (i.e. strings, bools, arrays, objects, etc). If you mean schema, then no you are not required to follow any schema. The only requirement is that you don't begin property names with an underscore.