London map
Get a map of London. On paper, or as a download. Many tourist guides have a folding one in the back.
Make a list of places you want to see and things you want to do. Try to give them some importance, from ‘essential’ to ‘definitely’ to ‘would be nice’.
If there’s a trip out of London, eg day trip to Windsor, that’s one day of your holiday sorted already. Yes, you can buy railway tickets in advance, and get them in paper form or (often) as a QR code on a smart device. You can order them beforehand and collect them at a ticket machine at (any) station, but you will need your payment method (ie card). Buying tickets in advance is generally cheaper than on the day.