Write trigger for primary contact for account object

I have written a trigger where if the user attempts to make a 2nd related contact on the account object a Primary Contact too, an error is thrown.  I am trying to add syntax to my trigger so if the user adds a related contact to an Account that currently has 0 related contacts, this new contact will automatically become the primary contact.  This is the code I have so far:

Trigger PrimaryContactTrigger on Contact (Before Insert, Before Update) {
    List<Id> actIds = New List<Id>();
    List<Contact> comingCons = New List<Contact>();
    List<Id> conIds = New List<Id>();
    If (Trigger.IsBefore && (Trigger.IsInsert || Trigger.IsUpdate)) {
        For (Contact Con : Trigger.New) {
            If (Con.IsPrimary__c == TRUE) {
                actIds.add(Con.AccountId);
                conIds.add(Con.Id);
                comingCons.add(Con);
            }
            }
        }
        List<Account> allRelatedAccounts = [
                Select Id, (
                        Select Id, IsPrimary__c
                        FROM Contacts
                        WHERE IsPrimary__c = TRUE
                        AND Id != :conIds
                )
                FROM Account
                WHERE Id = :actIds
        ];
        For (Contact EveryCon : comingCons) {
            For (Account EveryAccount : allRelatedAccounts) {
                If (EveryCon.AccountId == EveryAccount.Id && EveryAccount.Contacts.size() > 0) {
                    EveryCon.addError('THERE is already a primary contact for this account');
                }
            }
        }
    }