Simple but effective security for iOS to ASP.NET JSON service?

I am developing an iPhone application that GETS data from my ASP.NET web service that returns serialized JSON strings. My method is as follows:

[WebMethod(Description = "Set GPS Location")]
[ScriptMethod(UseHttpGet = true, ResponseFormat = ResponseFormat.Json)]
public string SetGPS(int userId, double lat, double lng)
{
    ...
    return cord.SerializeToJson();

}

Ignoring my temporary anti-pattern of returning a string, what would be a simple, but effective way to provide a security mechanism to ensure that communication between the client and server is secure?

I feel a bit at a loss in my transport to the iPhone and I don't know w开发者_如何学JAVAhere to start.

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