Help with background in tile based game

Im using Tony Pa’s Actionscript 2 tutorials to make a side scroller game similar to Mario (but you have a gun instead of jumping on enemies)

In order to make the background u hav to split it up into tiles, but i only know how to use tiles 0,1,2,3,4,5,6,7,8,9. When i try to use 10,11,12,etc it wont work. And i dont know how to set it up to use a,b,c etc. Is there any way to do this?, could i hav these background objects (like trees, bushes, etc) be called upon seperately and placed on the stage?

Any help is well needed and thanked, please explain throughly, im not very good at actionscript

-thejoeknows