in pokemon black and white 2, which button takes me to the main menu?

What you are asking for is typically called "Soft Resetting" and for DS games this is typically achieved by holding down the L and R shoulder buttons as well as Start and Select (for 3ds games it's Start or Select). This will return you to the startup screen.

Once at the startup screen (where the game display's the legendary and tells you to press start), there is a separate button combination to start the process of deleting your save; press Up, Select, and B at the same time, and then follow the onscreen dialogue, as it will ask you a number of questions to confirm if you want to erase the save, and a 'wrong' answer will abort the deletion process.

Alternatively, most emulators will have some sort of menu option that will manually reset the ROM, after which you can perform the in-game method to delete your save. Or you could close the emulator, and open up the folder where your ROMs (or sometimes, the 'BATTERY' folder in your emulator's folder) and delete the *.sav file for your ROM.