HttpRequest request = new HttpRequest();
    request.setEndpoint('https://test.salesforce.com/services/oauth2/token');
    request.setMethod('POST');
    request.setHeader('Content-Type', 'application/x-www-form-urlencoded');
    request.setBody('grant_type=password' +
        '&client_id=' + 'CLIENTID' +
        '&client_secret=' + 'SECRET' +
        '&username=' + EncodingUtil.urlEncode('USERNAME', 'UTF-8') +
        '&password=' + EncodingUtil.urlEncode('PASSWORD', 'UTF-8'));
    request.setHeader('Authorization', 'OAuth '+UserInfo.getSessionId());
    Http http1 = new Http();
    HTTPResponse response = http1.send(request);