I am  trying to create a new custom button on lead object which will assign assignment rule to lead.But something going wrong.i am new to coding as well as to salesforce.Please help:
code as below:

{!REQUIRESCRIPT("/soap/ajax/24.0/connection.js")} 
{!REQUIRESCRIPT("/soap/ajax/24.0/apex.js")} 

var leadStatus ="{!Lead.Status}";
var act = sforce.connection.query ("select id, whoid from task where who.type = 'LEAD' ");

var records = act.getArray("records");

if (leadStatus == 'Open - Not Contacted' && records.length !== 0)
{

var leadObj = new sforce.SObject("Lead");
leadObj.id = "{!Lead.Id}";
leadObj.Status = 'Open - Not Contacted';
leadObj.Country='India';
var result = sforce.connection.update([leadObj]);



var assignObj = sforce.connection.query("select id, name from assignmentrule where name = 'First' ");
var ruleid = assignObj.getArray("records")[0].get("id");
sforce.connection.assignmentRuleHeader = {}
sforce.connection.assignmentRuleHeader.assignmentRuleId=ruleid ;
var result = sforce.connection.update([leadObj]);




if (result[0].success =='false') {
alert(result[0].errors.message);
} else {
window.location.href = '/apex/your_page_goes_here?id="{!Lead.Id}"';
}


}

else
{
    alert('Check Lead Status and Schedule a meeting to continue');
}