Tinotenda, you don't have to immediately register it. Blogging is almost the same as your Facebook wall; it's not fully commercial until you decide to make it a commercial business. To answer your question, you can start receiving advertising income (from say, Google Adwords) without registering as a company.
In terms of tax obligations, they only fall due after you hit a certain threshold of revenue, or if you have to sell advertising to a large business here that wants you regularised for tax before they can pay you.
