The basic requirements for starting a Clan War are that you're in a clan with at least 10 members and that you are a Co-leader or a Leader.
If you still can't start a war because your Clan War icon is greyed out, or you are getting a message that says "Clan Wars are still loading, please try again", you probably need more bandwidth. Loading a Clan War involves transferring a lot of data, so we recommend using a strong 3G/4G or WiFi connection with a reliable network.