How can I read data from firebase which i am manually added into firebase database

Actually i want to implement Autoincrement id ...for that i went to the database & add childName with variable count =0.
Now when i insert data into posttable...i want to fetch that count from mannualy added & add 1 & store that value in post_id... & then update the count....so next time at the time of inserting count will be 1 & go on same in each inserting..
but i don't have idea to read that count.
help me with that...