If my layover in Beijing is less than 24 hr. and I like to do sightseeing in the city,

Rashmi, if you will go to a third country or region rather than the last one you come from after staying for less than 24 hours in Beijing, you can enjoy the 24 hour direct transit in Beijing, and then no entry permit will be needed.
You just need to apply for a free temporary stay permit at the immigration officer and then you can leave the airport to do sightseeing in the city.