Hi Swathi,

>> https://astreait.com/Lead-Conversion-In-Salesforce/

As mentioned in the above link, you can add the user creation apex snippet in the trigger handler class.

I am adding the snippet present in the above link below for quick reference:
 

trigger TriggerOnConvertLeads on Lead (after insert, after update) {
    List<lead> leadList = trigger.new;
    Set<ID> leadIds= new Set<ID>();
    for(lead leadRecord : leadList) {
        if (leadRecord.Status == ‘Qualified’ && leadRecord.isConverted == false){
              
             leadIds.add(leadRecord.Id);
        }
    //system.debug('size'+ leadIds.size());
    if(leadIds.size()  > 0){
              ConvertLeads.LeadAssign(leadIds);
    }
}
ConvertLeads class

Public class ConvertLeads
{
   
    public static void LeadAssign(Set<Id> LeadIds)
    {
       LeadStatus CLeadStatus= [SELECT Id, MasterLabel FROM LeadStatus 
       WHERE  IsConverted=true Limit 1];
      
        List<Database.LeadConvert> MassLeadconvert = new List<Database.LeadConvert>();
        for(id currentlead : LeadIds){
                Database.LeadConvert Leadconvert = new Database.LeadConvert();
                Leadconvert.setLeadId ( currentlead );                
                Leadconvert.setConvertedStatus(CLeadStatus.MasterLabel);
                Leadconvert.setDoNotCreateOpportunity(TRUE); 
                //you can remove this line if you want to create an opportunity during  conversion 
                MassLeadconvert.add(Leadconvert);
        }
        
        if (!MassLeadconvert.isEmpty()) {
            List<Database.LeadConvertResult> lcr = Database.convertLead(MassLeadconvert);
        }
    }}

For creating a user record you can check the below link for reference: https://developer.salesforce.com/forums/?id=906F000000092p2IAA

Let me know if it helps you and close your query by marking it as solved so that it can help others in the future.  

Thanks.