' You need "using" a lot System.* namespaces
   Namespace Whatever
       Class RequestHelper
           Public Sub makeRequest()
               Dim method As [String] = "GET"
               Dim url As [String] = "https://api.cardmarket.com/ws/v1.1/account"
               
               Dim request As HttpWebRequest = TryCast(WebRequest.CreateHttp(url), HttpWebRequest)
               Dim header As New OAuthHeader()
               request.Headers.Add(HttpRequestHeader.Authorization, header.getAuthorizationHeader(method, url))
               request.Method = method
               
               Dim response As HttpWebResponse = TryCast(request.GetResponse(), HttpWebResponse)
               Dim doc As New XmlDocument()
               doc.Load(response.GetResponseStream())
               ' proceed further
           End Sub
       End Class
       
       
       ''' <summary>
       ''' Class encapsulates tokens and secret to create OAuth signatures and return Authorization headers for web requests.
       ''' </summary>
       Class OAuthHeader
           ''' <summary>App Token</summary>
           Protected appToken As [String] = "bfaD9xOU0SXBhtBP"
           ''' <summary>App Secret</summary>
           Protected appSecret As [String] = "pChvrpp6AEOEwxBIIUBOvWcRG3X9xL4Y"
           ''' <summary>Access Token (Class should also implement an AccessToken property to set the value)</summary>
           Protected accessToken As [String] = "lBY1xptUJ7ZJSK01x4fNwzw8kAe5b10Q"
           ''' <summary>Access Token Secret (Class should also implement an AccessToken property to set the value)</summary>
           Protected accessSecret As [String] = "hc1wJAOX02pGGJK2uAv1ZOiwS7I9Tpoe"
           ''' <summary>OAuth Signature Method</summary>
           Protected signatureMethod As [String] = "HMAC-SHA1"
           ''' <summary>OAuth Version</summary>
           Protected version As [String] = "1.0"
           ''' <summary>All Header params compiled into a Dictionary</summary>
           Protected headerParams As IDictionary(Of [String], [String])
           
           ''' <summary>
           ''' Constructor
           ''' </summary>
           Public Sub New()
               ' String nonce = Guid.NewGuid().ToString("n");
               Dim nonce As [String] = "53eb1f44909d6"
               ' String timestamp = (DateTime.UtcNow.Subtract(new DateTime(1970, 1, 1))).TotalSeconds.ToString();
               Dim timestamp As [String] = "1407917892"
               ''' Initialize all class members
               Me.headerParams = New Dictionary(Of [String], [String])()
               Me.headerParams.Add("oauth_consumer_key", Me.appToken)
               Me.headerParams.Add("oauth_token", Me.accessToken)
               Me.headerParams.Add("oauth_nonce", nonce)
               Me.headerParams.Add("oauth_timestamp", timestamp)
               Me.headerParams.Add("oauth_signature_method", Me.signatureMethod)
               Me.headerParams.Add("oauth_version", Me.version)
           End Sub
           
           ''' <summary>
           ''' Pass request method and URI parameters to get the Authorization header value
           ''' </summary>
           ''' <param name="method">Request Method</param>
           ''' <param name="url">Request URI</param>
           ''' <returns>Authorization header value</returns>
           Public Function getAuthorizationHeader(method As [String], url As [String]) As [String]
               ''' Add the realm parameter to the header params
               Me.headerParams.Add("realm", url)
               
               ''' Start composing the base string from the method and request URI
               Dim baseString As [String] = method.ToUpper() & "&" & Uri.EscapeDataString(url) & "&"
               
               ''' Gather, encode, and sort the base string parameters
               Dim encodedParams As New SortedDictionary(Of [String], [String])()
               For Each parameter As KeyValuePair(Of [String], [String]) In Me.headerParams
                   If False = parameter.Key.Equals("realm") Then
                       encodedParams.Add(Uri.EscapeDataString(parameter.Key), Uri.EscapeDataString(parameter.Value))
                   End If
               Next
               
               ''' Expand the base string by the encoded parameter=value pairs
               Dim paramStrings As New List(Of [String])()
               For Each parameter As KeyValuePair(Of [String], [String]) In encodedParams
                   paramStrings.Add(parameter.Key + "=" + parameter.Value)
               Next
               Dim paramString As [String] = Uri.EscapeDataString([String].Join(Of [String])("&", paramStrings))
               baseString += paramString
               
               ''' Create the OAuth signature
               Dim signatureKey As [String] = Uri.EscapeDataString(Me.appSecret) & "&" & Uri.EscapeDataString(Me.accessSecret)
               Dim hasher As HMAC = HMACSHA1.Create()
               hasher.Key = Encoding.UTF8.GetBytes(signatureKey)
               Dim rawSignature As [Byte]() = hasher.ComputeHash(Encoding.UTF8.GetBytes(baseString))
               Dim oAuthSignature As [String] = Convert.ToBase64String(rawSignature)
               
               ''' Include the OAuth signature parameter in the header parameters array
               Me.headerParams.Add("oauth_signature", oAuthSignature)
               
               ''' Construct the header string
               Dim headerParamStrings As New List(Of [String])()
               For Each parameter As KeyValuePair(Of [String], [String]) In Me.headerParams
                   headerParamStrings.Add(parameter.Key + "=""" + parameter.Value & """")
               Next
               Dim authHeader As [String] = "OAuth " & [String].Join(Of [String])(", ", headerParamStrings)
               
               Return authHeader
           End Function
       End Class
   End Namespace