I have to write a blog on any topic! Would be interesting ?

I really have no idea.

It is you who should decide on what topic to blog about.Whatever the topic is ...try to be unique and pour in your ideas..without copying it from the internet/books.Think in what you are confident about...some may be quite confident in technology genre while some may be interested in makeup items/cooking recipes, you can also choose a blog about reviewing anything...be it a product/service/film/songs...people will definitely search for reviews before they buy the product or watch the film.So you will get many visitors for your blog.Also there are travel blogs..wherein you can narrate about the places that you visit with specifications of the boarding/travel facilities etc...There are many genres/topics to blog about..think and start with any..Keep posting and updating your blogs to attract readers...make it topic specific and blog clearly and with the correct information.Do not mislead your readers.If it is related to the recent trend then chances are there that you may get more traffic.
If you are looking for a free service to start your blog then Wordpress,Weebly,Blogger would be good.Once you get enough traffic then you can get a personal domain name and host it using XnYnZ.com and become a proud website owner.
Blogging is a good way of jotting down our ideas....this can be done as a hobby or we can monetize the blog and earn through it.

It depends on your life style. if you love traveling, try to create a travel blog. If you are a mom and wanted to share experience and tips. Try blog for moms.

Traveling these days is the most popular and fun thing to do that attract people towards it. I think you should try it.