The key is definitely priority. Arm training is fun and easy and doesn't make you hurt too much. Deadlifts, squats, and cleans are hard and cause great pain and take effort. You can't come into the gym Friday before happy hour to pump up on these, you can't squat or deadlift effectively with little sleep or a hangover or with 15 minutes to dedicate to a workout. people don't do these lifts because they're hard and taxing (which is why they work) people you meet will make every excuse not to do them such as, they hurt your knees, you don't need them, I only wanna look good, etc....It is all BS, so many people are so small because all they wanna do is train arms, and bench big numbers (even if they get burried and require being rescued by their spotter every set). Remmber that you'll get the most progress by doing the hardest and mosr avoided lifts.
You're young and getting started, don't fall into the trap that so many guys your age do. The guys who only train arms and walk around in wife beaters holding their arms out to the side like they're lats are too big to put them down and they weigh 150lbs and are weak as piss and the only people who are impressed with them are middle school girls.
Also, don't turn into a guy whose routine looks like this:
mon.
1.) Flat bench :135x10
185x5 (plus 5 with spot)
225x2 (with ass in air and add 5 reps cause spotter picked up the weight)
275 x burried (attempted to put ass in air, spotter rescued, you think you lifted the weight)
315x totally burried (ass in air, spotter almost tore something getting the weight off you, but a couple 15 yr old kids saw you with 3 plates and now think you're tough)
2.) every conceivable type of flye from d-bells to cables to machines (usually 2 hours worth)
3.) 1 hour of every conceivable type of biceps curl on every apparatus in the gym
Tuesday-Thursday Off
Friday pump up arms in a shirt thats 3 sizes too small while you are wearing cologne. workout only takes 5 min because you can't miss happy hour.
Saturday-Sunday Off
I don't mean this as a flame, but you're young, and while you are starting training for the same reasons a lot of young people start training, PLEASE don't be stupid. You are SMALL at 140, and when you see some jackass who is small at 170, but looks big to you, you'll wanna follow his every move, and most likely he'll follow the above listed workout. If I can steer 1 person in the right direction and help them understand what this is all about, I will be happy. If you wanna be thick, work your whole body, do compound movements, and focus on weaknesses.....if you're serious and want some program/diet advice, don't hesisate to ask.......Eat big and lift big and you'll be big, when YOU'RE big, your arms will be big too.