HI Swathi,
Try the code mentioned in http://theblogreaders.com/automatically-convert-lead-to-account-contact-and-opportunity-using-apex-trigger/

trigger LeadAutoContactConverter on Lead (after insert) {
 
LeadStatus convertStatus = [Select MasterLabel from LeadStatus where IsConverted = true limit 1];
 
List<Database.LeadConvert> leadConverts = new List<Database.LeadConvert>();
for (Lead lead: Trigger.new) {
if (!lead.isConverted && lead.Status == 'Open' ) {
Database.LeadConvert lc = new Database.LeadConvert();
String opportunityName = lead.Name;
 
lc.setLeadId(lead.Id);
//lc.setAccountId(AccountId); // you can set the AccountId instead of create a new account and contact is created the mention account
lc.setSendNotificationEmail(false);
lc.setOpportunityName(opportunityName);
//lc.setDoNotCreateOpportunity(true); // Optional to create a Opportunity
lc.setConvertedStatus(convertStatus.MasterLabel);
leadConverts.add(lc);
}
}
if (!leadConverts.isEmpty()) {
List<Database.LeadConvertResult> lcr = Database.convertLead(leadConverts);
}
}

Test Class:

@isTest
private class TestLeadTrigger {
static testMethod void TestLeadTrigger() {
Test.startTest();
Lead l = new Lead(FirstName = 'FName', LastName = 'LName', Company = 'Test Account', Status = 'Open', Email='info@theblogreaders.com');
insert l;
system.assertEquals(l.FirstName, 'FName');
test.stopTest();
}
}

Hope this helps you. Please mark this answer as best so that others facing the same issue will find this information useful. Thank you