need some urgent help,
I am very new to ssalesofrce and I am fully confused with lightning. I have a javascript button to validate some fields and then will allow to take PDF of a visualforce page. Now I need to migrate this JS button to use in Lightning. Please help me with this. 
Here is the JS button.

{!REQUIRESCRIPT("/soap/ajax/38.0/connection.js")} 
{!REQUIRESCRIPT("/soap/ajax/38.0/apex.js")}
 
        var PurchaseOrder = sforce.connection.query("SELECT Id, isPDFGenerated__c, ItemsCount__c, POReleaseDate__c, TotalQuantityPercentage__c FROM PurchaseOrders__c WHERE Id ='{!PurchaseOrders__c.Id}'");
        varPOrecords = PurchaseOrder.getArray("records");

        if(varPOrecords[0].isPDFGenerated__c == 'true' || varPOrecords[0].isPDFGenerated__c == true){ 
        alert("PDF already generated.\n\nPlease contact your Salesforce Admin to generate again."); 
        }
        else if(varPOrecords[0].ItemsCount__c == 0){ 
        alert("Purchase Order cannot be generated without items."); 
        }
        else if(varPOrecords[0].POReleaseDate__c == null || varPOrecords[0].POReleaseDate__c == ''){ 
        alert("Please enter Release Date."); 
        }
        else if(varPOrecords[0].TotalQuantityPercentage__c != 100){ 
        alert("Delivery Terms Total Quantity cannot be less than 100%"); 
        }                        
        else{
        if(confirm("Are you sure, you want to generate Purchase Order PDF?\n\nClick OK or Cancel.")){
        var result = sforce.apex.execute("GeneratePOPDF","saveGeneratePdf",{poId:varPOrecords[0].Id}); 
        window.location.reload();
        }
        }