How to set the correct projection ?

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Hi guys, I'm Alessio.
I've got a problem with MapServer Projections mechanism.

I've got the entire world map in Shape format.
In my project I've got a grid with Height=2xWidth and I need to load in each 
grid cell the appropriated world portion and after draw points at specifics 
lat-lon.
At this moment, I can load the entire world in the grid but the points I draw 
are in wrong positions.

I think because the map projection is wrong.
I don't know how projection is loaded because I don't understand the 
projection mechanism.
In my mapfile I've got:
"proj=latlon"
"ellps=wgs84"

Anyway, in some web sites I found a projection list, and I think that the 
correct one maybe the "Equirectangular", but I don't know how can I set it.

Can someone help me ?

Thanx in advance and excuse me for my bad english

Alessio