Hi,
If you obtain or buy a certificate from a CA and install on your ASA your PC will (hopefully) recognize the cert as trusted when you connect and the warning messages regarding untrusted certificates will not appear.
If you still get the warning messages check below re: DNS and CN/hostname.
If you issue a self-signed certificate you can still get round the warning messages, but you will need to configure the following.
1. Create a public DNS record to resolve the name of the ASA to the public IP that the AnyConnect Client will connect on. eg. HQFW01.domain.co.uk
2. When you issue the certificate the CN name must meet hostname. eg. CN=HQFW01.domain.co.uk, Hostname=HQFW01.domain.co.uk
3. You need to install the certificate in the 'Trusted Root' part of the certificate store on every remote device using AnyConnect.
Hope this helps.