You have not indicated for citizens of which country your question about registration in Belarus. Considering that the majority of users of Subtletties citizens of the Russian Federation answer: Russian citizens are not required to stay up to 30 days in Belarus.

If you are interested in registration regimes for citizens of other countries, then if you are going to stay longer than 5 days, you need to register. To do this, you need a passport, a migration card, a medical insurance policy.