How to decrypt a string encrypted with HMACSHA1?

I'm an encryption novice trying to pass some values back and forth between systems. I can encrypt the value, but can't seem to figure out how to decrypt on the other end. I've created a simple Windows Forms application using VB.NET. Trying to input a value and a key, encrypt and then decrypt to get the original value. Here's my code so far. Any help greatly appreciated. Thanks.

Imports System
Imports System.IO
Imports System.Security.Cryptography
Imports System.Text

Public Class Form1

    Private Sub btnEncode_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnEncode.Click
        Dim hmacsha1 As New HMACSHA1(Encoding.ASCII.GetBytes(txtKey.Text))
        Dim hashValue As Byte() = hmacsha1.ComputeHash(Encoding.ASCII.GetBytes(txtValue.Text))
        txtResult.Text = BytesToHexString(hashValue)
        hmacsha1.Clear()
    End Sub

    Private Sub btnDecode_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnDecode.Click
        '???
    End Sub

    Private Function BytesToHexString(ByVal bytes As Byte()) As String
        Dim output As String = String.Empty
        Dim i As Integer = 0
        Do While i < bytes.Length
            output += bytes(i).ToString("X2")
            i += 1
        Loop
        Return output
    End Function
End Class

HMAC-SHA1 is a one-way hash, not a bidirectional encryption algorithm. You can't decrypt it. I don't have time to provide full encryption code here - it's a complicated topic, but Barry Dorrans' "Beginning ASP.NET Security" would give you a good starting point. (Only some of it is ASP.NET-specific.) You could also watch his DDD talk on the topic.