Setting values to database:
DatabaseReference mRef =  database.getReference().child("Users").child(userId).child("Booking");
                    mRef.child("Restuarant").setValue("Value to be set");
                    mRef.child("Table Number").setValue("Value to be set");
                    mRef.child("Number of guests").setValue("Value to be set");

getting values:
public String getRestuarant() {
        // Access current User information
        // Firebase information
        FirebaseUser user =  mAuth.getCurrentUser();
        String userId = user.getUid();

        mDatabase = FirebaseDatabase.getInstance().getReference().child("Users").child(userId).child("Booking").child("Restuarant");
        mDatabase.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                restuarantString= dataSnapshot.getValue().toString();
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
                Toast.makeText(MapActivity.this, "Issue connecting with database", Toast.LENGTH_SHORT).show();
            }
        });
        return restuarantString;
    }