We know that Miitomo and the new My Nintendo loyalty programme launch on 17th March in Japan - the trouble is that we don't yet have a firm release date for the Americas and PAL territories. They're both coming in March, but as can be the way with smart device apps - and some Nintendo initiatives - the launch could come at relatively short notice.
If you want to get in early on My Nintendo goodness, then, you should make sure that you create your Nintendo Account ASAP. The perk being offered is a Platinum Coin / Points (the reward currency that'll primarily be linked to smartphone apps and services) which will be redeemable for goodies within Miitomo. Though the upcoming social app doesn't require a Nintendo Account (and related membership in My Nintendo), Nintendo seems keen to promote both as working hand in hand.
If you haven't already done so, then, there's still time to get signed up. You can sign up for a Nintendo Account on the official website here, then when logged in visit the Miitomo preregistration website.
If you want a step-by-step guide and a breakdown of what the Nintendo Account / My Nintendo and Miitomo are all about, we wrote up a handy guide a little while ago.
If you're already signed up and pre-registered all you can do is wait. Nintendo's new loyalty programme and debut smart device app are due before the month is out.