I think it would help to know if you like colour and, if so, what colours do you lean toward? Looks like you have clients come in so are there any colours that might be a turn off for them? As for windows -- do you mean window coverings?

Nothing says "library" like a beautiful chesterfield sofa! Have you considered maybe replacing the chairs to make a real statement? check out this leather "Cipriano" for example BANANAS !

Maybe a lighter color. Like a light warm gray? It seems like the dark wood trim and dark furniture actually darkens the room up a bit. So if you do a lighter color on the walls it will make it more open.

The dark molding around the a/c unit draws the eye, it might be better to paint it the color of the wall, or of the a/c front.

With only one picture it is hard to tell, but if there is space, try turning your desk at right angle to the wall, along the window wall with the chair closest to the window.

How should I begin?

Shd I use a wallpaper?

What is opposite your desk? I think that's the only wall we haven't seen. it must be the one below in the ceiling fan photo. With the style of your windows and color of the trim, I think I'd do the walls with a specialty treatment to get the Old World plaster warmth going. You may find a wallpaper that replicates that look, if you like the look an prefer paper. The photos would tell me to turn the desk to float, centered on the window so you can look out. Your chair would face the window with your back to the bookcases, although I might move those to the wall we haven't seen. You have one full bookcase and one mostly empty. Bring in some decor items and style them with 1/3 books, 1/3 decor and 1/3 open space.

Actually I want to convert it into a study room for my daughter so how should I begin renovating it?

My new library

Book shelf

A couch

The roof