If you're using iOS frameworks, your solution will involve using NSURLConnection or NSRLRequest. In-depth Apple documentation can be found here. In my experience, you want to wrap those API calls in your own client request that can build data transfer objects your application logic understands.
If you're looking at 3rd party frameworks to access a RESTful web service, take a peek at this answer for a number of iOS framework options.
For SOAP services, take a look here.
As for processing the data, it depends what kind of data your web service produces. There are several frameworks and libraries that can make it easy to consume XML or JSON.
In my experience, the best book on the subject is Advanced iOS Programming.
