What is the most common mistake of beginners?
They try to get healthy before they start a new training and nutrition program.
First and foremost, your definition of healthy is your biggest mistake. Following the general guidelines is exactly that: general guidelines. They are tips made for the average Joe. However, if you are reading this, it means that you want more than the average Joe. You want abs, you want muscles, you want to perform.
The average Joe wants the easy way. Bit of cardio and little to no weights, the less effort, the better it is. They want to see the weight on the scale go down, a few inches here and there disappear and voilà. They think they can go back to whatever they did, expecting to keep the weight off, like magic. They also believe the old myths, doing weights make you bulky, thinking they gain muscle fast like some genetic freaks. They think that eating meat gives you big muscles, same as for supplements. They rather go natural. (Cue eyes rolling in the back of my head.)
If you recognized yourself, you are your biggest limiting factor. You want the easy way out. Nothing is achieved while being comfortable. There will always be sacrifices in order to achieve greater health. Being comfortable and lazy is what got you out of shape in the first place.
If you invest right and make the necessary efforts at the very beginnersing, you will only have to adjust a few months in order to live healthy for the rest of your life.