How to write and read a simple 3 digit number

Sorry, I'm new to this and need a fast solution, so I'm asking for your help.
I have a USB card reader, a reader/writer with a software and some Mifare classic 1K cards.
I just need to write numbers from 001 to 999 to the cards and when I swap the cards in the reader it reads the 3 digits number.
Right now the reader (USB emulates keyboard) reads only the cards UID.
Is there a simple step by step guide for this?
In which block and sector I must write?
What do I do with the Key A?
Thanks for your help in advance!