I am a composite c1 noob myself, but one thing that I did and it seemed fairly straight forward was to understand how the contact form that comes with the Venus starter site works.
The Venus template contact form is driven by the function found in Functions > Razor > Composite > Form > Contact Form.
You can edit that function and you can see how it's implemented. Inside that function you have access to the form data and you can do pretty much whatever you want with it, since it's C# and Razor.
Additionally, you can create a custom Global Datatype
and optionally an associated Page Datafolder
to save your data, or have access to it outside the form function. These will have to be bound to the form function.
You can get the data submitted from the form using the automatically generated functions (should be located under Functions > All Functions > YourNamespace > YourDatatype). I've used the GetMyDataTypeXml function for what I was needing and it worked without issues.
You can read more about data functions here: http://docs.composite.net/Data/Data-Centric-Functions
Some examples of using the GetMyDatatypeXml function can be found here: http://docs.composite.net/Data/Data-Centric-Functions/Getting-Data-as-XML
HTH!