HttpRequest req = new HttpRequest();
req.setMethod('POST');
req.setHeader('Content-Type','application/x-www-form-urlencoded');
req.setEndpoint('https://login.salesforce.com/services/oauth2/token');
req.setBody('grant_type=password'+'&client_id=xxx'+'&client_secret=xxx'+'&username=username'+'&password=password+token');

Http h = new Http();
HTTPResponse res = h.send(req);

String token = '';

if (res.getStatusCode() >= 200 && res.getStatusCode() < 300) {
    Map<String, Object> m =  (Map<String, Object>)
    JSON.deserializeUntyped(res.getBody());
    token = string.valueOf(m.get('access_token'));
} else {
    system.debug('### error: '+res.getBody());
    system.debug('### endpoint: '+req.getEndPoint());
    system.debug('### statuscode: '+res.getStatusCode());
    system.debug('### status: '+res.getStatus());
}