public class HttpCalloutJob {

public final String consumerKey = '****';
public final String consumerSecret = '****';
public final String username = '****@nihon.com';
public final String password = 'MySfPassword****';
public final String reqbody = 'grant_type=password&client_id='+consumerKey+'&client_secret='+consumerSecret+'&username='+username+'&password='+password;

public void getCalloutResponseContents() {


    Http h = new Http();
    HttpRequest req = new HttpRequest();

    req.setBody(reqbody);
    req.setMethod('GET');
    req.setEndpoint('https://***.lightning.force.com/services/oauth2/token');

    HttpResponse res = h.send(req);

    OAuth2 objAuthenticationInfo = (OAuth2)JSON.deserialize(res.getbody(), OAuth2.class);

    System.debug('Body: ' + res.getBody());

    if(objAuthenticationInfo.access_token!=null){
        Http h1 = new Http();
        HttpRequest req1 = new HttpRequest();
        req1.setHeader('Authorization','Bearer '+objAuthenticationInfo.access_token);
        req1.setHeader('Content-Type','application/json');
        req1.setHeader('accept','application/json');

        req1.setHeader(???);   //I don't know, what I need to put inside
        req1.setMethod('GET');
        req1.setEndpoint('https://***.lightning.force.com/services/apexrest/jobShow');
        HttpResponse res1 = h1.send(req1);
        System.debug('RESPONSE_BODY '+res1.getbody());
    }
 }

public class OAuth2{
    public String id{get;set;}
    public String issued_at{get;set;}
    public String instance_url{get;set;}
    public String signature{get;set;}
    public String access_token{get;set;}
}
}