I would like write a sample application on NIOS which has BSP support of UART , which will receive or send data over uart on interrupt basis.
what are the apis i should use for the same and configuration i should do in the bsp settings to achieve the same
Nios has an IP that is 16550 compatible.
The 16550 and 16450 are chips that have been used for decades in PC's, you can find tons of information about these chips, their registers and their interrupts. Also if you use this IP you can analyse the Nios II C source code that presents a very good base to do what you want to do.