Here's my moment to shine!
At the airport, look for the signs for the Tube. Get on that and take the Picadilly Line into the city. You'll be on it for at least an hour or so because Heathrow is way out.
Kings Cross St Pancras is on the Picadilly line, and that's pretty close to C'well, but you might want to check to see what the nearest station to your brother is. Chances are, you'll have to change at Kings Cross anyway.