Best way to get from JFK to Laguardia airports?

I am a first time traveler per say ive never had to switch airports before and new york city is going to be my first place. my plane lands at 3:50 at JFK and i fly at LaGuardia at 7:40. i need a safe, cheap, and fast way to get from JFK to LaGuardia i have heard shuttle buses work but i am not sure how to go about getting on one or knwing what stop to get off at. i am 18 and a female obviously so safety is on the top of my lists. any help would be appreciated. but only serious comments, this is my time and kind of my life at stake here!!