can anyone tell me about making grid manualy

ok, i wish it was as simple as you would hoped.

there are many ways to do it, but prob the simplest i can think of is , you create a single plane and this plane is 1 of your squares of the grid , u make a prefab out of it, then u simply create it 20 times by 20 times. manually speaking this will take for every, so a small script with 2 loops should do the trick.

i believe there is a few "grid" assets on the asset store but i think they are paid, but i beleive they employ the same system of cloning each little square.

once the square form a grid ( your square should be textured with borders ) u can put OnMouse event onto a script attached to the prefab. like this each tile will know when the mouse is over it.

This is just a calculation problem. Lets say that the grid as 100 x 100 squares which means, including the edges, it has 101 lines. So to make things easy, make the plane 100 units by 100 units. If you are using the build-in plane, then scale it to 10 x 10 (since its default size when at (1,1,1) is 10 x 10). If you are using the CreatePlane editor script, you can just size it to 100 x 100. After resizing move the camera and/or adjust the field of view. Now all the coordinates will be whole numbers...easy to calculate and easy to find the nearest one if required. You could position the plane so that the bottom left corner is at the origin. This would make world coordinates and the coordinates of the intersections the same. If you set the center of the plane at the origin, you will need to offset coordinates by 1/2 the plane width. Note the mouse lives in Screen space, so you will need to use Camera.ScreenToWorldPoint() to find the World coordinate of your plane.