selecting multiple records

I have a form a single "main" record is displayed and a
list of other records on a subform. Multiple subform
records will match to the one main record. I need to be
able to have the user select multiple records on the
subform so I can set this match. As of now I can only
select records that are next to each other.

I also need to be able to calculate a total of a field in
the selected records and compare it with a value on the
main form. How would I reference the selected records or
get a count of how many records are selected?

Thanks for the help.
steve.