Starting on the right foot - don't make these beginner mistakes.
Are you a beginner to strength training?
Embrace the process of learning and developing the skill FIRST and you will improve your results and your longevity.
Learn HOW to lift
As obvious as this sounds, this is the most neglected part of beginning resistance training. Too often, boredom, the desire to avoid difficult things, and lack of education get in the way of proper training. Regardless of your goal, you need to start with a simple, low volume, low variation program to get the basics down.
This isn't some dogmatic soapbox about what type of squat to use, or that you must train with barbells the first day in the gym. What I mean is understanding how to move your body under load, safely. This includes the Valsalva, joint timing, position, proprioception, ROM, and load selection.
Doing isolation exercises will never teach you how to manipulate your body under load, you must do compound movements. Squats, presses, and deadlifts all done with free weights, and with sets low enough in reps that fatigue doesn't interfere with proper execution but still allows for adequate practice. A few sets of five work very well for this.
This is a critical point in a new trainees career and is no place for hubris or greed. Record your sets, choose proper loading, hire a coach, EDUCATE yourself. I understand that not everyone has extra money to hire a coach and that's fine, there are good resources out there for new trainees but for every good one, there are 1000 terrible ones. To cut through I offer free form checks and the first two weeks of training completely free with no obligation.
Feel free to reach out and ask questions.
Don't do TOO MUCH
Choosing the correct amount of WORK to do in any given session is another common beginner mistake. Stress, recovery, adaptation are the three things required to make progress. As you become more trained you require more stress to effectively disrupt homeostasis to spur a positive adaption.
However, when you are untrained you don't need much work at all to start this process off. This is one of the biggest things people get wrong. You do not need much stress as a beginner. The best analogy of this process I've ever seen is from Mark Rippetoe in his book Starting Strength. Think of the process of getting a tan, you don't go sit in the sun for 40 minutes the very first day, because you will just burn (the stress is too HIGH) and you won't be able to sit in the sun again in until you are back to as pale as you started (baseline).
So, what do you do? You sit out for 10 minutes on day one, then day two you sit out for 12 minutes (progressive overload), and then day three for 15 minutes, and so on until you are as crispy and brown as your wildest dreams dared be.
The same thing applies to exercise. Doing eight different "chest exercises" on day one of the gym is so much work that your body is so shocked it will struggle just to return to baseline, forget any type of actual positive adaptation. The typical "body part" split that is common is not an appropriate place for a beginner to start.
Two to three full-body sessions comprised of a few properly loaded compound exercises stress your entire body to elicit a positive adaptation, in strength, hypertrophy, and neuromuscular efficiency, and can be repeated frequently enough to allow for small increases in stress, creating a framework for ACTUAL progress to occur.
Changing things for no reason
Someone new to training does not benefit from a wide variety of exercises or training variables. You must give yourself the opportunity to become proficient in the core lifts and to learn what HARD feels like. This takes some time, depending on how athletic and aware of their bodies they are it can take a month to several months to learn.
No type of programming lasts forever, but when making changes things should be done in the smallest amount possible. Think along the lines of going from sets across to a top set with drop sets, or going from one set of five to two sets of three. These minimum effective dose changes allow progress to continue and allow you to know exactly what change worked or didn't work.
Completely throwing your program out the window because you missed a rep on bench is like throwing your car away because the oil needs changed.
Be humble and be a student
You wouldn't expect someone who just started at your job to be able to execute the required work with proficiency in the first few days. Resistance training is no different, it's a learned skill. Embrace the process, be willing to learn, and take the time to get the basics down and you will be far ahead of the majority of the people who set foot in the gym.