Japanese Grammar...once again...?

I was practicing Japanese with Rosetta Stone, and I am now somewhat dealing with formalities and even more grammar. However, I am confused. It is when I begin to use "wa" and "no". There was a phrase, "Bokutachi 'wa' otokonoko desu." We are boys, I believe. BUT, next to the picture was a picture of a mother and her daughter, and the phrase "Watashi 'no' musuko desu." What is causing this change? And why? I have also noticed the use of "ga" as well. When does "ga" come into play? And one more tidbit, shichi, which is 7, comes nani. How is this so? Rosetta Stone is amazing when it comes to helping you remember, but sometimes it is hard to comprehend the context of a situation. Any help is GREATLY appreciated. Thank you.