I plan to create a main tree named users which will include the name different users used as username. So, from each username will be included their data e.g. Full Name, Address, Phone No.
I want to know how to get each user's data when they log in on their profile.
First of all i suggest you spend some time getting familiar with firebase by reading the Firebase Guide (Link to old Firebase Guide). Everything you need to know to answer your own question is available there. But for simplicity i will put an example here:
Lets start with security, here are the basic firebase rules you need for this example: (source: Understanding Security) (old source: Understanding Security)
I will skip the actual user creation and logging in and focus on the question about storing and retrieving user data.
Storing data: (source: Firebase Authentication) (old source: User Authentication)
The resulting firebase data will look like this:
And last but not least the retreiving of the data, this can be done in several ways but for this example i'm gonna use a simple example from the firebase guide: (source: Read and Write data) (old source: Retreiving Data)
EDIT: Added example of return data
So when you are logged in as user twitter:123 you will get a reference to the location based on your user id and will get this data: