At 18 you should be able to get fit just by reading about it! To succeed at the bleep test you do need some stamina, so start off with longer slower work on the running. Say 5 miles at an 8m/mile pace. When comfortable at that then alternate between a longer run at a slightly quicker pace, and some fast rep work, say 400m reps x 10, building to 15. If you put the work into the reps you'll build speed, and a six miler at 7 min/mile will build the stamina. Work up to three of each/week.
How about some swimming as well? Junior #2 thinks nothing of 70 lengths in about 50 mins.