I am not sure what you mean by "I created a clan".
You need to have the call of duty app to set up the clan.
Give the following link a read to see if it helps:
Because you are new to clans I suggest watching the clan video here:
For understanding the cod app you should read this:
To create a clan tag you need the cod ghost app,,,,if you have the app you need to go to the my clan page...once there in the upper right hand corner is a helmet with a gear in it that is the manage clan tab hit that,,,, once on the manage clan page you will see where you can create a clan tag edit clan name , it may take a bit to get a clan tag if you try making one that already exist... After you have made a clan tag on the app you can go to the game and create a soldier hit Where you pick unformed patches and such you will see towards the bottom the clan tag tab click on that and then you can select to wear your clan tag if you want to wear your clans tag .