Import Data records through plugin

Hi Experts,

My requirement is,I want to import data from Excel to CRM.

While importing data,CRM should check whether record is existing or not.If it is not there then it will create new.If record is already there(Here based on name it checks the record) it should update the value.For this I want to write one plugin.So how can I write plugin?And on which message I need to add step(Create/Update?).

Thanks in advance.