Here are some things to do and/or have:
The vets #'s, both emergency and off hours
Iodine to dip the end of the umbilical in
Twine to tie the umbilical off
Flash light with new batteries
Foaling stall with a lot of soft straw in it
Blanket to rub the foal off or wrap them in if it is cold
I am sure others will be able to add to this list. Here is a very helpful web site that you can read and it may help you too..