Try to finish the course and see if the content is valuable for your purpose. If it isn't keep learning in another site. 

That's a broad and unspecific request. I can tell you that KA's teaching has very little about form validation, but there are a few things about it in the HTML/CSS/jQuery course.

Try starting here:

https://www.khanacademy.org/computing/computer-programming/html-js-jquery/form-processing-jquery/a/process-forms-with-jquery-intro

Thank you so much for this guiding link. I start learning. And this is very helpful for me. I hope I will be able to input forms validate for my site saudi national day 2021. Appreciation.