I've done the following code but I can't figure out how to get an output to come out;

I used a structure to store both strings and doubles, and then created an array using that datatype.

However I can't work out how to create an output. It wont let me convert either so I haven't figured this out. Any tips would be appreciated.

Public Class Form1

Structure Salestaff 'Created a Salestaff structure in order to have more than one datatype and call them
    Dim Name As String
    Dim Sales As Double
    Dim Salary As Double
End Structure

Dim Staff(3) As Salestaff 'Array for staff
Const Commission As Double = 0.015
Const Base As Double = 450
Dim Salary As Double
Dim DisplaySales As String

Private Sub btnAddsales_Click(sender As System.Object, e As System.EventArgs) Handles btnAddsales.Click

    If lstStaff.SelectedItem = "John King" Then
        Staff(0).Name = "John King"
        Staff(0).Sales = Val(txtSales.Text)
        Staff(0).Salary = Base + (Commission * Staff(0).Sales) 'Should apply formula
    ElseIf lstStaff.SelectedItem = "David Smith" Then
        Staff(1).Name = "David Smith"
        Staff(1).Sales = Val(txtSales.Text)
        Staff(1).Salary = Base + (Commission * Staff(1).Sales)
    ElseIf lstStaff.SelectedItem = "Tim Cook" Then
        Staff(2).Name = "Tim Cook"
        Staff(2).Sales = Val(txtSales.Text)
        Staff(2).Salary = Base + (Commission * Staff(2).Sales)
    ElseIf lstStaff.SelectedItem = "Ewan McDonnell" Then
        Staff(3).Name = "Ewan McDonnell"
        Staff(3).Sales = Val(txtSales.Text)
        Staff(3).Salary = Base + (Commission * Staff(3).Sales)
    End If

End Sub

Function DisplayStaff()    'Should take Staff info and allow it to be outputted
    Dim Staffreport As String
    Staffreport = Staff
    Return Staffreport
End Function

Private Sub btnDisplay_Click(sender As System.Object, e As System.EventArgs) Handles btnDisplay.Click
    Dim Password As String
    Dim Passwordcounter As Integer

    Password = "manager"
    Passwordcounter = 0

    If InputBox("Please enter your password") = Password And Passwordcounter <= 3 Then 'Create an Inputbox and enter password manager
        DisplayStaff(ByVal Staff As Salestaff)                                          'Should pass values in parameter to function
    Else : Passwordcounter += 1
        If Passwordcounter >= 3 Then
            MsgBox("You have had three attempts")
            MsgBox("Please try again")
            InputBox("Please reenter your password")
        End If
    End If
End Sub

A couple of things, first the function isn't declared properly. A function declaration has to include a return type and a variable to hold the info being passed to it, Function DisplayStaff(Staff() as Salestaff) As String.

Second to put the properties of the structure into a string you have to iterate through the array, then convert each property, if needed, and add the string to your output string. Something like this should work:

Function DisplayStaff(StaffArray() as Salestaff) As String 'Should take Staff info and allow it to be outputted
    Dim Staffreport As String = ""
    For Each s as Salestaff in StaffArray
        Staffreport += "Name: " + s.Name + " ; Salary: " + s.Salary.Tostring.Trim + " ; Sales: " + s.Sales.Tostring.Trim + vbNewLine
    Return Staffreport
End Function

To call it you need to assign the return output to a variable. something like this:

Dim Report As String = DisplayStaff(Staff)