Here is a basic tutorial: WCF (SOAP) Service With Federated Authentication
Then you can look at samples by Dominick Baier and Brock Allen here: Thinktecture.IdentityModel.45.
Also you can try to use Visual Studio tool for adding STS reference and point it to Azure ACS.
AFAIK endpoint Uri is something like this for client certificate authentication:
https://myacsnamespace.accesscontrol.windows.net/v2/wstrust/13/certificate
More samples are located here: Windows Azure AD Access Control (ACS) Code Samples
If you have questions, consider asking them as separate and more specific topics.
