How to get average of a continuous value updating every one sec

Local variable values do not persist between calls to the method. Each time your Timer1_Tick method is called, all of the local variables will be set to their defaults.

You also don't have any code to retrieve the data - you're just adding 0 to the total each time.

You need to store the list of values in a field. Each time you add a new value, remove any values which are too old before you calculate the average.

A simple class to store the values might looks something like:

Imports System
Imports System.Collections.Generic
Imports System.Linq

Public Class RollingAverage
    Private ReadOnly _limit As Integer
    Private _values As Queue(Of Double)
    
    Public Sub New(ByVal limit As Integer)
        _limit = limit
        _values = New Queue(Of Double)
    End Sub
    
    Public Sub Add(ByVal value As Double)
        If _values.Count = _limit Then
            _values.Dequeue()
        End If
        
        _values.Enqueue(value)
    End Sub
    
    Public ReadOnly Property Count As Integer
        Get
            Return _values.Count
        End Get
    End Property
    
    Public ReadOnly Property Average As Double
        Get
            Return _values.Average()
        End Get
    End Property
End Class

Store an instance of that class in a field, and use it to calculating the averages:

Private _averages As New RollingAverage(60)

Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
    
    Dim data As Double = ... TODO: Get the data from somewhere ...
    
    _averages.Add(data)
    
    If _averages.Count = 60 Then
        Average = _averages.Average
    End If
End Sub