Start developing with the Developer tier, it doesn't come with an SLA but has a very low price which is suitable for development environments.
When you're ready to make your API's available for consumption from the internet then deploy another instance which is using the lowest tier that is suitable for you. You only tend to use the Premium SKU for them advanced scenarios (private networking, etc). For Internet accessible, i'd suggest you start on Basic. You are able to upgrade the SKU type easily, so don't worry too much.
For more information on the APIM SKU's, see : https://learn.microsoft.com/en-us/azure/api-management/api-management-features