Hello, I wanted to throw a birthday party here for my daughter I was going to call the number listed for city hall but they are closed hopefully I can get an answer here do I need to make reservations for this park or is it whoever gets there first?

Westminster shelters list the stipulations on whether or not you can reserve a certain structure for a party. The location includes a phone number to call to make the reservation. There are several parks that do not offer Reserve tables as well so check the location for the sign

You should try to reserve it first. If there are no reservations for any given day, it is whomever gets there first.