1) Leaning backwards
A natural mistake people make when first learning to ski? Leaning backwards. This is a natural response as you learn to pick up some speed while skiing. However, when you lean backwards, you become prone to falling (exactly what you're trying to avoid)! If you learn forward, you'll have more of your weight on your foot and subsequently on more of the ski; you'll be able to steer and control the skis much more easily.
2) Wearing the wrong gear
Decathlon has plenty of gear--jackets, pants, mid layers, base layers--so you have no excuse to go skiing in just jeans and a coat. You'll need something that is not only warm but waterproof. You need the proper clothing with the proper thermal protection to make sure you don't freeze while skiing, especially if you're new. Don't forget--it's easy to take a layer off than to put a layer on, especially if you're too stiff to do so! Make sure you find the right fit so you'll be cozy but unencumbered.
3) Being underconfident OR overconfident
There's a healthy middle. Maybe you've done a similar sport before, or maybe you just felt like you were really pro on your first bout. We all start somewhere--and a black diamond run is not it! It's good to graduate from smaller runs and move onto larger ones when you feel ready.
By the same token, don't let the risk of falling scare you away from skiing. In fact, being scared will almost certainly make you more prone to mistakes and falls. If you're new, expect to make mistakes and forgive yourself for the ones to do make and KEEP GOING! If you're too fearful, your muscles will lock up and it will be too hard to continue skiing and to continue learning. If you need to take a break, feel free.
4) Being oblivious to your surroundings
Be sure to look around while skiing. You may be inclined to look at your skis to make sure they're in the right position, or you may be distracted by other things going on in the piste. Pause and train yourself to look ahead--you want to see WHERE you'll be going so you can intuit where your skis are by how they feel and know what to anticipate from the upcoming terrain. Looking ahead ensures you'll see any unexpected upcoming obstacles, such as a fallen branch or another skier coming in fast up ahead.
When you're in a new area, scope out and see what others are doing and get a sense of what to watch out for before you begin in earnest. There are people of varying skills, make sure you give others ample room to ski whether they are more skilled or less skilled than you are.
5) Going too slow
It's easier to feel the ground undernear you when you're going at the right speed. If you're too slow, you will not be able to get a "foot feel" on the terrain underneath you, becoming unable to detect changes small in the ground (elevation, consistency). Go at a comfortable but slightly challenging pace so you can get a good feel as to how the skis respond to the ground below you and then respond in kind so you can adjust to the run (instead of the run adjusting you!)
6) Forgetting your goggles
Sunglasses won't cut it. You need a pair of goggles that won't fall off and leave you exposed to the cold. No need to buy the most expensive ones on the market. We've got you covered--we have goggles for all sorts of weather so you can ski no matter what life throws at you.
As always, be sure to have fun out there! It'll be a challenge at first but the reward pays off.