Hi rohiranh, welcome to the Guild. You could take a look at this article, which lists various software packages some of which are free or relatively low cost. Generally, Gimp and Inkscape are the best free options. I've never used Inkcarnate though I hear people talking about it more and more.

I like the colour palette you've chosen in your map, though I'm not completely sold on some of your land mass shapes, but maybe that's just me. You're off to a nice start however, and I'll look forward to seeing your next update.