well you'd attach the hair to a plane and then make it short (actually thinking about it, does it have to be hair? or can it be fur? fur would be more suited to this!) either way, make the hair/fur nice and short and then adjust the amount of it so that you get a good coverage. Then from there you need to select the hair/fur and add dynamic fields to it (parented to moving objects if you want them to move around and disturb the grass etc.)

I can't really explain it further without making a sample file (dont have time at the moment) but look in the manual for the stuff about hair and using fields.

Alan