Welcome to the site.
Tip #1 when writing stories: Make sure you write a complete story, no matter how much you want to share it with the world and release a demo. It doesn't have to be long, just complete.
Tip #2 when writing at CYS: Read the Help & Info section. Pay close attention to 10 ways to make better games, Captain Kiel's Rules Of Writing, and Creating Storygames: Not A Walk In The Park.
Tip #3: For the Boy vs Girl issue, as I see it, there are 3 options:
1. Write gender neutral. Find a way to not specifically make things for one gender or the other. This is the easiest solution.
2. Start writing "IF" statements in code. You can ask the reader if they are a boy or a girl at the start and then on every page where you need to refer to a "he" or "his," you can include "IF" statements in the code. This can be done, but will take some time, and you have to be very careful to catch every instance.
3. Write completely different paths. In other words, if the reader selects "boy," then they are going to get a completely different story (all different pages) than if they select "girl." This is likely the longest option, as it would require you to write a lot more and keep track of many different things. And you can be sure that when a reader reads you story, they will follow one path and likely play again to follow the other path anyway.
Have fun reading and writing here!