How do I connect to SharePoint with a WP 8.1 app

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SharePoint

REST

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Hello people!
I'm developing an app for windows phone 8.1 .
I want to read and write data to and from Microsoft SharePoint.
I'm using REST API and my problem is I have no idea how to get the access token needed in order to gain access to SharePoint.
This is what I have so far:

HttpWebRequest spRequest = (HttpWebRequest)WebRequest.Create(jsonRequest);
spRequest.Method = "GET";
spRequest.Accept = "application/json; odata=verbose";
//spRequest.Headers["Authorization"] = "Bearer" + accessToken;
HttpWebResponse endpointResponse = (HttpWebResponse)await spRequest.GetResponseAsync();
Stream dataStream = null;
StreamReader reader = null;
string responseFromServer = null;
dataStream = endpointResponse.GetResponseStream();
reader = new StreamReader(dataStream);
responseFromServer = reader.ReadToEnd();

Maybe my approach is all wrong.
I'll appreciate any insights, please help I spent a week on google trying to figure out how to do it.
Thanks.