Iam basically a salesman , I sell green energy to Brazil, on behalf of British Companies, do I need to register as exporter or do I have to have a license to export?
I’m not sure if I understand your situation correctly – when you say that you are exporting "green energy", do you mean that you are exporting a product, or technology? Also, are you selling the product yourself, or on behalf of other people? Who is billing the overseas client? yourself or a client?.
If you are selling products, and you are the person who is billing the overseas client., you are the exporter, and you will need to register with HMRC for an EORI (Economic Operator Registration & Identification) number. If you are not selling the products in your own right, but on behalf of another UK company, they will be the exporter.
As to whether you, or your client, need an export licence, that will depend on exactly what the product / service is. Some technology may be liable for export controls. If you are in any doubt, you can check the UK Export Control Lists by going to: https://www.gov.uk/government/publications/uk-strategic-export-control-lists-the-consolidated-list-of-strategic-military-and-dual-use-items-that-require-export-authorisation