Informants are the folks that give you contracts to complete. As you level up you unlock new informants. Just keep completing contracts and this will unlock. For an informant to count, you must pay them their initial cost they ask when you first click on them. Sometimes it is a cash requirement, other times it is a credit requirement. This should first be attainable around Level 10.
*GLITCH* Some players have reported this achievement does not unlock for them, even though they have more than 10 informants unlocked. You can either wait for a patch or reinstall the game and try again. Doing so will erase your progress however.