If you consider the tilemap rows/columns as "world coordinates" then you can use a simple formula to convert x and y values to tilemap rows and columns:

Tile_Column = X / Tile_Size
Tile_Row = Y / Tile_Size

Note that the division operations should be using integers to automatically round the answer down for you. If you're using real value types you need to use math.floor().

For screen to world coordinates, you first have to define where the screen is in the world:

View_X: The X position of the screen's top-left corner in world coordinates.
View_Y: The Y position of the screen's top-right corner in world coordinates.

Then you can convert on-screen coordinates to world coordinates (i.e: for a mouse click) with the following formula:

World_X = View_X + Screen_X
World_Y = View_Y + Screen_Y

Finally, you can take the World_X and World_Y values and put them into the first formula to get which tilemap column/row the coordinate is inside.

If your grass tile's position in the tilemap is <Gx,Gy> then its pixel position in the game world will be:

tile_worldX = Gx * Tile_Size
tile_worldY = Gy * Tile_Size

Then you subtract the position of the screen's top-left corner from these coordinates to get their on-screen pixel position:

tile_screenX = tile_worldX - view_x
tile_screenY = tile_worldY - view_y

This assumes that the screen's top-left corner is 0,0 in your viewport. Sometimes game engines will move 0,0 to the middle or bottom-left of the viewport so you may have to compensate for that difference.