Can i start writing under a subject of my choice?

Go to "Write" Pick a channel. Pick a subchannel. Then scroll down the page to find the paragraph that begins with "Can’t find your title?" . It will have a link for you to use. However you really should look first to see if there are existing topics; people earn more from popular groups (aka topics).

The other members are correct but do strive to find the best place for your article first. I am a site steward at Helium - message me if I can be of any help to you.