Hello Folks, 
               I am still exploring on Integration part in salesforce but for now i had come accroseed a requirement that from salesforce i needed to send the opportunitylineitem record from salesforce to external system So here is my code 

Global class IntegrationClass {
    /*
    @HttpGet
    global static List<opportunity> doGet() {
        RestRequest req = RestContext.request;
        RestResponse res = RestContext.response;
        String BuyerId = req.requestURI.substring(req.requestURI.lastIndexOf('/')+1);
        Account result = [SELECT Id, Name, Phone,Bi_Id__c, Website FROM Account WHERE Bi_Id__c = :BuyerId];
        
            
                String acc = result.Id;
                List<Opportunity> opp = [Select id, name, accountId from Opportunity where accountId =:acc ];
                 return opp;
            
           
        
        
    }*/
    public static void sendData(){
        Http http = new Http();
        HttpRequest request = new HttpRequest();
        request.setEndpoint('https://hookb.in/6JqjXyjp2RsoO0ro3nP7');
        request.setMethod('POST');
        OpportunityLineItem opp = [Select id, name,ListPrice,UnitPrice, Opportunity.Id,Opportunity.CloseDate, Opportunity.Account.BI_ID__c from OpportunityLineItem  Limit 1];
        System.debug(JSON.serialize(opp));
        String name = opp.name;
        System.debug('!!!' +name);
        Decimal listPrice = opp.ListPrice;
        System.debug('!!!' +listPrice);
        Decimal unitPrice = opp.UnitPrice;
        System.debug('!!!' +unitPrice);
        Date closeDate = opp.Opportunity.CloseDate;
        System.debug('!!!' +closeDate);
        
        request.setBody('{OpportunityLineItem opp}');
        request.setBody('{"Name":"any--JUL 2022 Mobile", "ListPrice" : "123.00", "UnitPrice" : "123.00" , "CloseDate" : "2022-07-01 00:00:00"}');
        request.setHeader('Content-Type', 'Application/Json; charset = UTF-8');
        System.debug('!!!'+request);
        HttpResponse response = http.send(request);
        System.debug('!!!'+response);
    }
        

}

 So I will be getting opportunity line item record which is been needed
to sent and serializing it so the way I sent that seriallized record is the correct way to pass under request body or there is some other way around. 
Please let me know or any suggestion