Create a new trigger on opportunity which will set the stage name is prospecting and close date is 15 days from today.

trigger OpportunityTrigger on Opportunity (before insert) {
    if(Trigger.isBefore && Trigger.isInsert){
        for(Opportunity oppObject: Trigger.New){
           oppObject.StageName ='Prospecting';
           oppObject.CloseDate = System.today().addDays(15);
        }
    }
}