Create an insert trigger on Lead, which will update rating as hot, whenever new Lead record is created.

To the above task create and trigger on Lead Object, and paste the below code to see results.

trigger Leadtrigger on Lead (before insert) {
    if(Trigger.isBefore && Trigger.isInsert){
        for(Lead eachLead: Trigger.New){
            eachLead.Rating = 'Hot';
        }
    }
}

If you have already trigger on Lead, use below code.

if(Trigger.isBefore && Trigger.isInsert){
        for(Lead eachLead: Trigger.New){
            eachLead.Rating = 'Hot';
        }
    }