Who are you flying with? Heathrow is a big place so know which terminal and leave sufficient time, two hours should be enough, but if you have anxiety or want to shop/eat, three might be better.
Most airlines now have apps and your boarding pass is within the app. I usually screenshot mine and use that.
Pay attention to the departure boards in the lounge, they will indicate what time your gate will be posted. When it is posted, make your way there.
When you board the aircraft, let one of the cabin crew know you haven't flown before. It is their job to ensure safety and they will be happy to help you out.
Pay attention to the safety briefing and read the safety card card in your chair pocket. I have flown many times and I always do this, I don't understand why people don't. They will be the ones getting trampled in an emergency 😈
If it's a short flight, it could be a bit bumpy, don't worry about it, the higher you fly, the smoother the air. The pilots are trained for this.
Most of all relax, and if you have a window seat, enjoy the view.