I did not have experience with webMathematica. For arbitrary input fields in CDF see this blog Using Mathematica Enterprise Edition to Create Professional Apps, Tools, and Reports
With respect to your comment - "I have no experience at all in building a webpage"
- I think you do expect some knowledge if you wish to "eventually integrate the GUI on a webpage". Distinguished feature of CDF is that it requires minimal knowledge of web development. The video I linked to is for non-specialists.
On the other hand you may want to wait till Mathematica Online
- you can apply for testing there.