How do I update from Snow leopard when it will not allow access to App store

I have had my Macbook Pro for some years from new and didn't update, it therefore became unusable as the system was now extinct.

After stripping it down and reformating , it appears to work.

But I still need to update, unfortunately it won'

t allow access to the App store for me to do this, and sends an error code if I try to update ? can someone help?