Accessing record ID in the Lightning component is very simple. Just use following things.

1. Implements force:hasRecordId interface in the component as follows.

   <aura:component implements="flexipage:availableForRecordHome,force:hasRecordId" access="global">

2. Define an aura:attribute as follows.

   <aura:attribute name="recordId"
    type="String"
    description="The ID of the record to be displayed. Provided by force:hasRecordId interface."/>