To the OP.
The mains reasons you'll want to avoid steroid use for now is growth. Muscle growth and the rate of it's growth, your bodies natural growth, and your brain development (growth).
As Dylan has touched on, you're so light.. especially for your height.. if you were to put on 20-30 lbs over 2-3 mos. (seems like a while, but it's not) you will ache like you couldn't believe. You won't be able to do anything normally and it may take six months to a year for your body to reach a physical homeostasis again. A point at which you feel neutral or comfortable. Your muscle will out grow your joints, ligaments, and tendons. You will be very injury prone, that is.. if you don't get injured mid cycle. This is no no #1.
Number 2, you are 18.. which rings a bell in my head.. the average male will still be physically developing until age 21. Meaning you are absolutely not 100% grown yet. If you start adding in hormones to your natural growth and your estrogen gets too high, bye bye growth plates. They will close, and you will lose 3 years of free, and natural growth from god. Wait till 21 for physical growth to be finished but don't think it's quite time to use AAS yet!
There is more.. The average male's brain will not be fully developed, full maturity will not have been met ON AVERAGE until 25 years old. Give or take 1-2 yrs. Meaning some people may reach full brain maturity at 27, others 23.. but not typically any less or any more. Wait till you're at least 25 to use AAS if you plan on having a 100% functionally mature brain (the most important muscle you've got)
The reason AAS gets in the way of these natural developments comes down to the very nature of their chemical make-up, and our bodies reactions to them. The most easily understandable reasoning is this..
Any exogenous (outside source) androgen introduced into the blood will completely shut down your Hypo-Thalamic-Pituitary-Testicular-Axis (HPTA for short). Meaning sections of your brain and all of your testicles will be shut down for the entire time the androgen is present. Why shut down the brain or balls while they are still in development?
Be safe, and don't use until you are of age. You have been warned.