Can someone help me how to get a icon in a fan club?

unless you created the club you cant change the icon on your own. make a pick on the spot asking other fans for their opinion and to give them a chance to submit new ideas as well. once the fans have chosen you can message papa or dave and they will change it
for more info (and i do encourage you to read it cause this is just the basics that i put down here) read this article:
link you will find your question answered at number 17!

and you should read the whole thing cause dammit its pretty good!