how to use testbench???

You need a simulation program (like Modelsim). The testbench creates an instance of your device under test (UART in your case) and at a minimum provides stimulus. It may also do some self checking. Use the testbench as your top level file in simulation.

I have two files. One is the uart_serial.vhd and the other one is tb_uart.vhd(testbench for my uart_serial.vhd). Set this tb_uart.vhd as top-level entity?

Correct - the testbench is what you select as the top level of your simulation