Okay. Don't freak out, here's what you're going to need to do (but keep in mind it's going to cost you about 30 pounds):
When you go through customs at Heathrow, take the Heathrow Express train, here is a link
to Paddington Station in central London. There are signs pointing to it everywhere as you leave customs. Do take this instead of the tube or the bus because the journey is only about 15 minutes, whereas the tube & bus will take *at least* an hour.
When you get to Paddington Station, look at the Tube map-- or better yet, check it out here
in advance to get yourself over to Liverpool Station. When you get to Liverpool, you are going to hop on the Stansted Express train, click here
. That journey will take about 45 minutes. You should get there in time to get in line to check in for your flight.
Good luck!!! B)