You will need to visit your nearest Telkom store with ID and proof of residence.
Please make sure the store puts the sim on the SmartBroadband Wireless tariff plan as 1 of my friends were put on the Telkom More tariff plan and purchased the 500gb for R50 (valid for 31 days) and could not use it and has to wait 30 days to change the tariff plan.
@Bond, upon getting your new SIM, insert it into a phone and dial *1# to get the number, save it in a safe place should anything happen to the new SIM.