Random 2D tile engine question

 void Start () {
     tiles[0] = grass;
     tiles[1] = dirt;
     
     Generate(0,0,20,20);
 }
 
 void Generate(int i, int j, int Width, int Height) {
     width = Width;
     height = Height;
     for (i = 0; i < width; i++) {
         for(j = 0; j < height; j++) {
             clone = Instantiate(tiles[Random.Range(0,tiles.Length)],new Vector2(i*tileWidth, j*tileHeight),Quaternion.identity)as GameObject;
         }
     }        
 }

Is this the way I should generate a random tile engine or is there a better way? How can I make is so I can edit the tiles and change their textures during runtime? Is there a way to lower say the amount of dirt that is generated so that there is more grass than dirt?