First, you need to get a license to be able to provide money transfer services. Also, most countries like the US will need a way to monitor the business to ensure that you are not money laundering.
Now, start your business once the license is ready. However, you need to decide if the business should be online or have a physical presence, or both. It is good to weigh the pros and cons of each option.
It is also good to create relationships with banks since your business will be transferring money to them.