Hi, I want to know how we can implement menu system. Basically I want that if I press certain button, the game is paused and user will be presented with inventory menu screen.

Is it implemented by using made panel bitmap to cover the screen? If yes, could somebody explain how to do it or the helpful website link? Thanks

with panels, yes
take a look at lite-c workshop. there are basics of working with panels, digits...
http://www.conitec.net/litec/