I will need more information about your setup and what you have tried so far to have any chance of knowing what's wrong. For instance, what FW have you programmed the chip with? Are you using a custom/third-party board or a Nordic DK board? nRF connect or some other Android app? etc..
Actually, I have started newly, I have to connect my android device with nrf52840 for my project, but now I cannot know anything how can I do this, how can I start?
First step is to program a Bluetooth peripheral sample application on your board to make it connectable. You may try to program one of the pre-compiled SDK examples as instructed in the documentation here: Getting Started.
