A simple google search for: mikrotik ddns script
(or better yet i like to use on google : mikrotik ddns script site:forum.mikrotik.com )
will find your answer.
here are some links from that search:
http://forum.mikrotik.com/t/dynamic-dns-update-script-for-no-ip-dns-for-router-os-v-6-7/73699/1
and
http://forum.mikrotik.com/t/dynamic-dns-one-script-to-rule-them-all/59088/1
Just be sure that whatever DDNS service you use, that your account stays valid on that service (ie some of them make you “check in” every X number of months, if you are on their free tier) – i suggest using mikrotiks builtin “/ip cloud” service , if it works for your network setup (ie you dont have the issues we talk about in this thread)