Http h = new Http();
HttpRequest request = new HttpRequest();
request.setEndpoint('https://login.salesforce.com/services/oauth2/token?grant_type=password&client_id=XXXXXXXXXX&client_secret=YYYYYYYYY&username=yourusername&password=yourpassword');
request.setMethod('POST');
request.setHeader('Content-Type', 'application/x-www-form-urlencoded');
HttpResponse response = h.send(request);
System.debug('response '+response.getBody());

{"id":"https://login.salesforce.com/id/00Dxxxxxxxx/005xxxxxxxxx",
"issued_at":"1278448832702","instance_url":"https://yourInstance.salesforce.com/",
"signature":"xxxxxxxxxx=","access_token":
"00Dxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"}