SetActiveTile - active tiles and co-ords in TGB some Q's

Hi all,
have worked through many tutorials and seen some posts, but I need more help ...
regarding parameters...
//"%tileObj = %obj.setActiveTile( "0 0", myActiveTileClass, myActiveTileDatablock )"

1) how do I determine the location of a tile do I use pickTile(xy) ? what is the xy .. tiles can cover more than one co-ordinate?
2) Same xy Q regarding setActive tile ...
3) How do I create a tile class ?

Sorry might seem easy to some, but am learning fast and have a prototype game working after buying 2 weeks ago ... just want to improve...
Making a turn based / real time game (make decisions and then it runs until time / action break --> eventually port to 3D engine) and will eventually hope to post some tutorials of my own for newbies...

Thanks Damian