I live in South Africa, but working for a offshore company.

You would declare this on your tax return under "foreign services rendered" You would then need to declare this income earned and pay the tax amount on it. SARS will calculate how much tax is owing based on the income earned and your deductions if you have any.

Will I have to register as a provisional tax payer, or can I do this as a normal tax payer?

You would have to register as a provisional taxpayer as you are not paying over PAYE each month due to working full time for a company which pays tax in SA.