Beginner that want understand

Hi all, I am a beginner with a trash English and I would start to program px4 code. I searched in the site for build one drone and now I have a pretty quad, but when I tryed to find some info on the site and on Internet in general. I founed only the guide called “Writing your First Application”, which didn’t have a lot of info. Can someone help me for this issue recommending me some sites or guides for learn and then develope something?
Thank you for the help

Maybe start here:

Good luck.