All trains from Waterloo and many from Victoria pass through Clapham Junction making it the UK's busiest station! It is a major interchange and may seem daunting at first but the platforms are labelled with the destinations they serve in addition to the indicators for individual trains.
It is best to change platforms using the huge footbridge (with lifts to all platforms) where there is space to stop and check which platform you require, the subway is usually too busy to stop without causing congestion.
Note that Clapham Junction is not served by an Underground line but London Overground lines use platforms 1 and 2.

In a quiet residential area of London, this Queen Anne house was the home of the writer and historian
Thomas Carlyle. The Victorian period decor, furniture, pictures and books are all still in place.
Open Mar-Oct, Wed 1100-1600, booking essential.
www.nationaltrust.org.uk
Bus 170 from stops A, C and E to Albert Bridge, about 15 minutes journey,
Daily, about every 10 minutes.
Buses also operate from Sloane Square and Victoria.
The second oldest botanic garden in the UK. The 4 acre peaceful garden contains many rare and unusual
plants. Medicinal plants and tender species are a speciality.
Open M-F & Sun 1100-1700 (1600 Fri).
www.chelseaphysicgarden.co.uk
Bus as above but to Chelsea Physic Garden, about 15 minutes journey.
Chelsea Physic Garden is ¾ mile from Sloane Square.
The story of the British Army from Agincourt in the 15th century to peace-keeping today.
Displays cover campaigns and battles plus the social history of the Army. Café.
Open WThFSSu 1000-1730.
www.nam.ac.uk
Bus 170 from stops A, C and E to opposite the museum, about 20 minutes journey,
Daily, about every 10 minutes. Buses also operate from Victoria.
The Museum is ½ mile from Sloane Square station.
Museum, Chapel, Great Hall, Courts and Grounds. Walking tours can be booked,
please see the website for details.
Open please see website.
www.chelsea-pensioners.co.uk
Bus as above to opposite entrance, about 20 minutes journey.
The Royal Hospital is ½ mile from Sloane Square station.
