Setting up the flash is extremely simple. The minimum requirements is 2 movieclips (3 for animated walking).
MOVIE CLIP ONE - The player
Draw what you want to be the idle stance of your player (It could even be a small circle) and make it into a movieclip - you don't need an instance name.
On the second frame of the MC, draw your character facing right (You could make this into a movieclip to have a walking animation)
MOVIE CLIP TWO - The background
This one is quite simple - make a picture into a movieclip (make sure it can loop horizontally) and make sure the registration point is on the far right.
Give this MC the instance name of BACKGROUND
In the movielip, duplicate the picture and put it on the right of the first one.
THE STAGE
Add a new layer for the background.
Merely place the player on the higher layer and the backgorund underneath.
Remember that you don't need to change anything in order for it to work (for instance, changing a variable every time you change the movie's size) as it gets all it's info on it's own.