Depending on where you purchased your domain from, you might already have a domain email address included in your domain package. It's worth checking with your domain provider if you are not sure.
If you don't have a domain email service, you can send an email from your own domain address using an external provider - the most popular of these include Google Workspace and Microsoft 365 both of which offer various paid plans depending on your business needs. Zoho is another which offers a limited but free alternative for domain email.
Depending on your email program, you may also be able to set up replies using the "Reply To" field.
Because of the differences between the many email programs available we are unfortunately unable to provide more detailed support for this. We suggest that you look for information on changing the "Reply To" address in the documentation for your particular email program. The option will probably be found under something like email accounts or account settings.
Alternatively, if you are using the software that they gave you when you joined them, you could contact the technical support department of your Internet service provider.