from nanning to Guangzhou can I book bullet train ticket online?

Yes, you can do it online via a agent like this website.

I,m new so can you teach me how

They have the train page, you input the cities and date, then you can get all the schedule that day. You just need to choose the one you want and click"book" then submit the order online, They will contact you by email to guide you. Not hard to do so, you can have a try.