Hi Battal,
There are several ways of handling this. An easy solution would be to add a Status attribute to the Quotation entity (enumeration with values New, InProgress, processed (or whatever values are required for your process)).
When a Customer submits a quotation, you can set the value to New.
For the Sales rep you can create an overview (eg on the homepage) with the Quotations with Status = New.
When the Sales rep opens the Quotation, you could trigger a microflow to change the value to InProgress and save the Quotation. Of course you will have to make sure the Sales Rep can get to the overall overview of Quotations (eg via a navigation item).
Like I said there are multiple solutions some of which are more advanced. You can also check out a notification solution as part of an Intermediate Level Learning Path. But that goes beyond first steps development.