Form CO. (BTW, we explain the procedure for registering an app in our new mini manual for app developers.) To file an application, you first need to determine which elements of the app are your original authorship. For example, if you only contributed some text and software code, and you licensed the rest, then you would only claim copyright (and seek registration) for what you created. You indicate that information in Form CO -- the all purpose copyright application -- in the section under 'authorship.' Later, in Section 4A of the form you must list the items for which you are not claiming copyright.
Pick Your Category. As with any copyright application you must establish what "category" of work you are registering. Most software programs are registered as 'literary works' - an anachronism dating back to the fact that source code is written in letters and numerals. However, if your app is primarily pictures, choose 'visual arts' work, and if it is a graphics-heavy product like a game, choose 'performing arts' work. Don't worry if your app seems to straddle two categories -- just pick the one that seems best.
Do You Need a Marketing Firm? The Dear Rich Staff doesn't know whether you should use a marketing firm. That's not our bailiwick. In any case it sounds expensive.
Can You Sell it Elsewhere? We don't see anything in the iPhone Developer Agreement that prohibits your porting from one mobile OS to another.
Will We Send You the Answer? No, we won't send you the answer so if you were hoping for an email response, it's going to get very Godot-ish waiting by your inbox. Sometimes, if we're not overwhelmed with managing our Netflix queue, scrutinizing credit card statements (Did we Skype to Czechoslovakia?), and monitoring our TransLink card, we do write back to people