Interacting pc and lpc2148

What do you expect? Ready made code where you just have to insert your 
name that solves the task and satisfies your teacher?
Just start and ask more specifically if you run into problems.

i wont say that i want the readymade code. just tell me how to do that
ie communication between pc and arm (sending messages in both direction)

The LPC214x controller offers USB and UART interfaces. For a start it's 
much easier to use the LPC's UART. If you PC does not offer a 
serial-port you can use a usb-serial cable available in nearly every 
computer shop.

You need an interface-IC like i.e. a MAX3232 between LPC and PC's 
serial-port. Just look into the schematics of LPC214x evaluation boards 
for hardware-details.

Example-code on reading from/writing to an LPC UART can be found in the 
net (i.e. in the sample-collection fro LPC213x/214x from NXP.com, 
example-code from Bill Knight in the LPC yahoo-group).

Once you are familiar with developing code for the LPC you can start 
using the LPC's USB-interface.