As seen in the preview, there is this bar at top with search on its right.

Add a menu to the “Topbar” area and it will appear.

https://en.support.wordpress.com/menus/

Thanks a lot Alex, I have few other things to ask should I ask here?

You could, if they are pretty “simple” ones. If there are any you think are pretty big, creating a new thread is often good so others can easily find by searching for title.

yeah these are simple ones. What should be size of Logo and header that I can upload? (Tell in Pixels)

By default, the logo would be 120px height. The double of 60px, which is how to show it in high resolution.

However you can set the height to what you wish in Theme Options > Styling. “Logo Max-height”. Set what you want there and then upload an image with the double height of that.

What can be width of logo and also tell height and width of header image, thanks a lot

It all depends on what styling options, screen resolution and website max-width that you use. Most image widths should work. By default, the header full-width image is 1380px wide.