Hi,
How do I arrange sprites and choose what layer they are on? My sprites keep going behind things and I can't see them.

You can use the “go to front” blocks and “go back (x) layers” blocks.
For example, if you want a sprite to be in front of everything,

when gf clicked //In sprite that goes in front
go to front 

when gf clicked //In all other sprites
go to front
go back (1) layers

to make sure that the sprite is in the front.

And if you want to make sure a sprite is always in the back, you can use:

when gf clicked
go to front
forever
go back (999) layers
end

For the always in the back…

when green flag clicked
go back (99) layers

99 layers is usually enough to push it to the back, if not, add another 9.

this should do:

when green flag clicked
go back (999999999999999999999999999999) layers

when green flag clicked
go to front

That's the way I'd do it…at least for the front part.

when green flag clicked
go back (however many) layers

To go back.

If sprite1 is at the back, then sprite2, then sprite 3 and then sprite4 at the front

In the script for sprite 1

when green flag clicked
go to front
show
broadcast [layer sprite2 v] and wait
broadcast [layer sprite3 v] and wait
broadcast [layer sprite4 v]

For each sprite from 2 to 4 add the following script using the correct sprite name

when I receive [layer sprite2 v]
show
go to front

To set the layer of a sprite to make sure it overlaps the ones you want, but underlaps others do this:

when Lower Sprite's Green Flag clicked ::hat ::color events // Make sure this is ordered correctly. Sets a sprite at the second layer. (or the go back number - 1 layer.)
forever ::color events
go to front ::color events
go back (1) layers ::color events
end

when Upper Sprite's Green Flag clicked ::hat ::color events // Sets it at the front layer.
forever ::color events
go to front ::color events
end ::color events