I am 18 years old. how much exercise do i need on a daily and/or weekly basis?

Exercise: A minimum of three days a week and at least half hour of aerobic exercise with some strength training also. It should be a lifelong part of your day.

Regular: Aerobic exercise several times a week is best to maintain health. What activity and how much depends on what you are trying to do -- stay fit, lose weight, train for a marathon, etc. A measure of "adequate" exercise is getting your heart rate significantly above its normal rate. Talk to your doctor about his/her specific recommendation based on your size, current activity and general health.

30 min most days: 30-45 minutes of aerobic type exercise most days of the week is the general rule. The recommendations seem to change monthly with every study that comes out. In fact you'll probably get 5 different answers on here depending on which study they read. The important thing is stay consistent with it which means trying different exercises and sports so you don't get bored. Also do some weight training.