Hi,
You can use FormsCentral to design a form, and save that form as a pdf fillable form. Or if you have an existing pdf, and want to use FormsCentral, you can refer to this FAQ, http://forums.adobe.com/docs/DOC-1391. Or if you want to make a pdf a fillable form in Acrobat. Please ask this in Acrobat forum.
Thanks,
Perry