Help: cost map

[ROSEO]

Hi users!

I'm a french user and I had GRASS4.1 since 4 months.
I work for an archeologic service.
I would like to create a cost map. I had 3 maps (slope, networks), I know
that in first, I must create cost surfaces with my 3 originals maps and then
make an overlay, and after to do r.cost.
But I'm stop in the first level, I don't know how to assign the value for
each pixel of my 3 original maps. And after for the overlay do I use r.mapcalc?

If somebody know how to do (and understand my poor english), please let me know
.

Thanks

ROSEO anne
6,quai st vincent
69 Lyon