Autodiscover is a means to help simplify the configuration of remote Outlook installations and mobile devices to use Exchange. Microsoft's verbose documentation about this feature is here.
The easiest approach to get over the autodiscover hurdle is to create a public DNS A Record for
autodiscover.mydomain.com that points to your SBS server's IP address. This can also be a CNAME alias, but I prefer the A Record. This can be requested or configured through your domain registrar or whomever manages your public DNS. That's really the bulk of the setup from the DNS side.
The other portion is to deal with your SSL certificate. If you browse to your webmail address and examinee the SSL certificate, you'll see what DNS names are configured. You may want to have the autodiscover names on the same cert. Again, a more involved process, but helpful in the long run.
Since this is an SBS, you may only have a single-name certificate. I'd check with the original implementor, but your alternative would be to create a fallback SRV DNS record instead. See this approach from Microsoft and additional notes here.
Even though autodiscover fails for you now, you should still be able to configure your mail manually. have you been able to get that far?