What terminal you are?
You dont have to be scared because you are genuine, you'll be sweating like a pig if youre not and if your carrying stuffs thats not permitted to bring in the UK.
Before you land, the stewards will distribute a card which you will fill up with infos and you will hand that over to the immigration desk.
Once you get through immigration, just go and dont hang about.
Make sure your luggage is alright. No missing wheels or not broken due to mishandling of the airline staffs. They'll give you instructions if its damage.
Just be attentive, you'll be alright.