To join a clan, you need to make sure that you are at least level 2 and have 6 Victory points (VP.) players using Apple devices cannot invite or join a clan which was created by an Android user or vice versa.
To find a specific clan, you can use the search field in the Clan section. Please make sure that you use the exact clan name to search.
Note that clan leaders set their own minimum level and VP for their clan, so it may be higher than 2/6 as described above.