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Unless you can go back in time and start your clan topic earlier, you can't ;) unless they change some things, then you can't go into other tiers for now

After you login in the forum, you go to the clan page
Like he said^^^ just press 1, 2, 3, 4 or 5
You can find a forum clan on the link which marine already gave above. If you want a clan on starladder, go to the 'Tournament' section on top of this page. To join a clan, first find a clan and click 'request to join' on the right. Please note that you will face a lot of generallisimos on Starladder and clan leaders are usually looking for the best players only. You can't have clan wars on Starladder, meaning you can, but it will not affect your clan points in any way and neither you wins and losses. And you will have to ask your opponents clan leader regarding time, map, ranks etc... and it will be a sort of "friendly" between you guys