How is the first node in a linked list determined?

In a linked list, the first node is determined by a reference to it, which is usually stored in a separate variable often called head. This variable points to the first element of the list. If the list is empty, head is equal to null.

Example in Java:

class Node {
    int data;
    Node next;

    Node(int data) {
        this.data = data;
        this.next = null;
    }
}

class LinkedList {
    Node head; // first node of the list

    void add(int data) {
        Node newNode = new Node(data);
        if (head == null) {
            head = newNode; // first node
        } else {
            Node current = head;
            while (current.next != null) {
                current = current.next;
            }
            current.next = newNode;
        }
    }
}

Thus, the first node is the one that head points to.