As a first time traveller, here are a few things done differently and would have only made life easy had I known them before hand :p
Getting out of immigration will take forever (close to an hour or 2)
Don't forget to activate international roaming! Or instead of getting roaming activated on your sim, simply walk into a store at a shopping complex and buy a sim enabling 3G for how much ever data required for the duration of the stay. It's cheaper and frankly easier.
Always halt on the right of the escalator and walk down the left. Always.
Keep a screen shot of the tube route handy to help you chalk out the path and time to the place of travel to the T.
Before crossing the street looking either way isn't enough, don't forget to push the button. I always forgot, sigh!
Do not fly Air India! It took me 36 hours to get back home when it should have taken me no more than 15 (including connecting). Quite a harrowing experience.
And don't forget to smileeeeeee :D it'll always brighten someone's cloudy and gloomy day :)