The terminals
Heathrow has four terminals numbered two to five. Which one you arrive at could seem random (it should say on your itinerary). Three (and sometimes two) is usually for the big US airlines (eg American). Five is generally for British Airways (BA).
Which you arrive at makes no real difference, but don’t assume your flight back is at the same terminal. Get up-to-date information on the day of the flight, and use airline or travel company apps if you can.
To get out of the terminal may involve considerable walking. Be aware also that terminal five has a transit system for the journey from the gates far away to the arrival halls. It’s usually packed, but there’s a long walk otherwise.
You will also, of course, have to pass through immigration and collect any bags you may have checked. This all seems a slow process (because it is), but you can get there eventually.
Now we can decide how to get to our destination.