I want to become an agent who sell properties online but I am not sure where to start so can anyone help me

If you have realestate credentials I see no problem with that. Set up web page explaining what you have and start from there. Give your phone number etc.

To sell real estate within the US, you will have to have a real estate license for the state the property is located at. Most will require you get a licesne and work under a Real Estate Broker for 1 to 3 years before you can be a broker. To operate your own company you will have to be a broker, You can sell though a broker as a agent but the agent will get part of each sale.

Also there are strict laws in each state even regulating what can, what has to be, and what can not be on a advertisement