I am a beginner. Can some one help me with this?

Assuming that the start of the video memory (video segment) is contained in ES and, address of the coordinate (70,13) is in DI, and that the used video mode has 256 colors, how do you change the color of the pixel in (70,13) to white. (one line of code)

In 256 colors modes you also need to setup a pallette.
So assuming that you have a linear black to white shades of gray with color 00 beeing black and color 0FFh beeing white:

to get the logical scan line length you can use VESA function 4F06h (see RBIL for details) or get it directly from CRT register 3D4, 13h (this value must then be multiplied by 8 for 256 color modes)