Should I start a clan?

It depends what level you are. If you are beginner, then you probably should find an alliance to join and not spend 500 gems on making your own alliance. The main reason is that joining an established alliance will give you all of the alliance benefits and resource help that it offers. Starting your own alliance from zero will make it hard to attract new members, unless you have established friendships in the game, willing to join your new alliance before you make it, I would not start a alliance.

You can find more information about joining an alliance here: https://www.appgamer.com/rise-of-civilizations/strategy-guide/joining-an-alliance