Which language(s) are you familiar with and, probably more important, which server is the project supposed to be capable of running on when completed.
A Linux/Unix web server will normally limit you to PHP with MySQL while Windows hosting will allow you to use ASP or .NET, but other options are out there like Perl/CGI, ColdFusion and Java, for example.
For a really simple solution you could use plain HTML with some clever form handling to process your 'orders' and calculate payment, VAT and delivery etc.
It would help to know whether you are expected to write an underlying database of products from which to choose or if you have items X, Y and Z and all you offer are quantities of those at fixed prices.
Get back with more detail and one of us will point you in the right direction - include the software you have available to you for writing your web and whether or not this is to be data driven.