Please find the details of lead conversion 

Trigger on Lead
Trigger on lead( after insert)
{
Get the trigger size (Integer)-

takelist of type <database.leadStatus>---this list is used to hold the leads that needs to be converted

Query the lead Status  Isconverted=true(where condition)[ id,Masterlable, should be fetched]
iterate For( i=0;<triggerSize;i++)
{
Create new instance of database.leadconvert 

set the following values to instance 

 set the leadID("id from trigger"),
notCreateOpportunity to(" true")
set convertedstatus(this value from the leadStatus Query)

after setting add it to the list of type<database.leadStatus>-declared at the top
}


//create Database.LeadConvertResult[] array—Shown below [you can use exact line shown below]

Database.LeadConvertResult[] leadResult= Database.convertLead(pass the listOfleads that needs to be converted,false);


//itereate the leadResult and check the system.debug to know the success or failure