Ok I've been training for 10+ years and I remember when I started at the age of 16-17 I was a weak skinny guy that couldn't lift the bar without weights for chest press. Read a lot about exersices, fitness magazines, workouts, know your muscle groups and how to isolate every muscle to work it out alone but also learn exersices that train several muscles at once.
Some important things because everytime we see new guys at the gym there're is a pattern of mistakes that repeats over and over:
- Stick to the basics- the basic exersices Probably seem too ordinary to some but are the most important, muscle building exersices , so no matter what you read on magazines and all kinds of fancy shit u see. Never Skip those Squats , Deadlift, Presses, Pull ups etc.
- Let your muscles Rest - my first few years I went to the gym 6 days a week , trained every body part 2 times a week used all type of supplements and the results were close to nothing , I couldn't belived how some of the "big guys" by then were getting such a nice results with lot less training. I followed a recommendation that changed my life , train each body part ONCE a week, but train it like you mean it. Then I started growing and I started seeing my body transforming. After a good workout and breaking down that muscle tissue very good the muscle have the rest of the week to recover and reconstruct that muscle tissue (hypertrophy) harder, bigger, meaner. Of course with proper nutrition and rest.
-Weak first- If you have a weak muscle or that part of the body that give you hard time, do it FIRST. For example if you have problems with your calves, and is legs day , start your workout with calves , since you are fresh and more motivated with more strengh you are going to push them to grow more than if you do calves after 2 or 3 leg exersices n specially those heavy squats.
-Rest between- Is important that you know your body and your resting time between sets what's maybe too little for one is too much for otherone, rest too much youll lose your pump rest too little youre gonna fatigue. Usually too maintain " the pump" I rest from 45-60 seconds between sets on regular exersices and about 2 min on heavy exersices.
-Don't let it settle- Change your exersises, routine, techniques every once in a while so your body don't stop growing getting used to the routine/exersices, this is also called plateau. I change every 3 months the exersices , and the muscle groups work together also help.

Ok this few things are important , I know there are a lot more things that I didn't mentioned ( sorry tired as hell falling asleep lol) also diet is super important but there're so many threads about it. An I'm sorry for my bad English , is not my first language so apologize for the typos .