avishkar,
You have to write the code, that's the fun part! Anyway, that sounds like a good project that you will successfully finish.
I don't think C++ is the right language for the project. There are a lot of issues that might needlessly distract you, and it would be hard to show something cool at the end. I would recommend JavaScript (a.k.a. ECMAScript). The cool part is that your program will be integrated into a web page that should be viewable in any browser. You can even put it in a web page hosted for free at places like wordpress.com so that the whole world can see and use it.
Have a look at this:
http://www.itworld.com/storage/254444/k ... r-languagehttps://www.google.com/search?q=learnin ... javascripthttp://jquery.com/http://code.google.com/p/flot/Beware! Keep it simple! Don't get lost in programming issues that don't have to do with your project! The air rifle problem sounds like you just need to allow people to plug values into an equation and have the program spit out an answer. That can be done with an HTML form to input the values and press a button (or have JavaScript detect the entries) and then show the answer on the page. JavaScript is exactly the right tool for such a thing.
As to something with fish, you could also create forms where people plug in values for common everyday fish calculations (I know nothing about such things, though) and have the form provide the answer. Temperature conversions, salinity, pH, I don't know what sorts of things are involved, but I suspect you do.
Good Luck!
Heinz