Exchange server external email configuration

I have installed exchange 2k3 on SBS2k3 server,my company has external email which is administor by external Web hosting Provider.(register.com).Now my company want to keep our own exchange server.i have domain name .local and smtp domain name .com i have dought how to point my exchange server for external users.is i need to register my mx for my exchange server? but where i have to do it ? from where i will get mx record public address .what i have to configured in my DNS and exchange server?
please reply me step by step .for DNS and exchange server.is i need to contact to my external mail provider? is they will do?
plz reply me…
ritesh