To enter the ELo ranking you need to be level 20 and to have a Elo compliant deck (no doubles and the sum of the levels of the characters must no be over 25, you can have 8 or more cards in your deck, but obvisously, 8 is the better way to start)
When you are Elo compliant, a pillz will appear next to your name in the game lobby. You still can be challenged by everyone, but you need to play against other Elo player for the fight to be registered for ELo rankings
When you begin in ELO, you will start with a value of 1000. Then, after each ELO battle, your value will go up or down based on the result of the battle and the ELO value of your opponent.
You should find helpfull information reading the Elo part of the game help.