Just an example.

For example, we will be posting some account info from salesfore to another salesforce org.I know there are other ways but we need to do it through user Oauth authetication in which we need to get the code from the other org and then we will be using that code for authorization purposes and then sedn it with the authorization url.

Firstly,
How do I get that code in Apex .
2)In the body of the below given code ,I have specified rant type,I do not have any code value to assign to the response_type becasue I believe it has to come 
from the other org and I do not know how to get it from the other code. I have client secret and cliet id  and I have specified the Redirect_URI as the Endpoint URL.


Can u please guide how to start to get code .
I am assuming I will use salesforce  endpoint URL and 'https://login.salesforce.com/services/oauth2/token' and then some how get code and then 

I will use salesforce authorization URL in which I will have to pass code value and other information .


Can u please help me with the intial step?


     
     String ClientId='xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx';
     String ClientSecret='xxxxxxxxxxxxxxxxxxxxxxxxxxxxx';
     HttpRequest req = new HttpRequest();
     req.setHeader('Content-Type','application/x-www-form-urlencoded');
     req.setMethod('POST');
     req.setEndpoint('https://login.salesforce.com/services/oauth2/token');
        
      req.setBody('grant_type=authorization_code'+'response_type=code'+'&client_id='+clientId +'&client_secret=' + clientSecret+'&redirect_uri='+redirectUri);
     
     
     Http http = new Http();
     HTTPResponse res = http.send(req);
     System.debug('BODY: '+res.getBody());
     System.debug('STATUS:'+res.getStatus());
     System.debug('STATUS_CODE:'+res.getStatusCode());