London is served by five airports.
Heathrow airport is the largest. It is straightforward to get from Heathrow to central London: https://www.heathrow.com/transport-and-directions/getting-to-central-london. The cheapest and easiest way is to take the Tube: the Piccadilly Line will take you into central London in about 50 minutes. If you are in a hurry, you could take the Heathrow Express. This is a fast train that runs between Heathrow and Paddington Station. Often you will need to get a Tube from Paddington so it can be quicker to just take the Tube from Heathrow.
Gatwick airport is another popular airport that is relatively easy to get to: https://www.gatwickairport.com/to-and-from/. The Gatwick Express train service runs regularly between Gatwick airport and Victoria Station. It is slightly cheaper to buy your tickets in advance from https://www.gatwickexpress.com.
London City Airport is the most central airport, but is quite small. It takes about 30 minutes to get into central London: https://www.londoncityairport.com/getting-here/to-and-from-airport
Stansted airport is located outside London but there is a regular Express train service into central London (Tottingham Hale or Liverpool Street Station) that takes about 45 minutes: https://www.stanstedairport.com/getting-to-and-from/
Luton airport is also outside London. Again, there is a regular train service into London (St. Pancras) https://www.london-luton.co.uk/to-and-from-lla. There is a short bus journey between Luton airport and the Luton airport train station.
If you are coming from central Europe, perhaps the easiest way to get to London is with the Eurostar: https://www.eurostar.com/.