Can we surely get money just by writing ebook?

I wouldn't say we surely can earn money but yes, you can earn some money through it. It could take time and unless you have a nice publicity going on for it, then money could be rolling in faster. I am sure you will make some money out of it but, seriously, I am not sure how much you can make. There is first time for everything. Just don't give up!

You don't make money by doing nothing... You need to write the ebook and promote it. I agree with the first response, you should write because you love to write not because you want to make money, you will fail unless you promote your work correctly, than people could like your ebook and buy it.