Being a rookie is the gym is exciting, but tough. You have so much potential to fulfil but the pitfalls are all to easy to fall down on. Here's three of the biggest mistakes I see beginners making (not to mention three I made for years too!).
1/ TOO MUCH WEIGHT, NOT ENOUGH FORM: Whatever the movement, you need to master the form first before you start banging up the weight. It will take time for your body to adjust to these alien movement patterns and practise to get a strong mind-muscle connection (the ability to squeeze, contract and ultimately feel the relevant muscle groups working during a movement).
2/ TAKING ENOUGH DRUGS TO OPEN A PHARMACY: Whether this be the legit off the shelf supplements (protein powder, vitamins, minerals, pre workouts etc) or the darker, less legal stuff; as a beginner you do not need a cupboard full of supplements. A basic protein powder, a decent multivitamin and fish oil and you're good to go.
3/ TRAINING FREQUENCY: Less really is more when starting out. Instinctively it's easy to feel like more equals better, but training 3-4 times a week will pay you back far more so than 6-7 days a week as you start out. Focus on quality over quantity. If you try to hammer yourself silly daily, then you're going to fall victim to mediocre workouts, sub par recovery or a big dent in motivation when you realise how tough it is to practically maintain that kind of routine.