My first pokemon game was yellow and I was so young when I started playing that I couldn't read. getting the game for my birthday is actually one of my earliest memories. That's the beginner's mistake right there, playing the game before you can read.
For some more specific examples:
It literally took me years to get past the first gym (starting with pikachu did not help)
When I finally did, I got a little farther in the game then reset. I did that a lot.
Later, I got crystal and was much more successful (partly because by then I could read. a little...), I almost beat the game, but I still did a lot of stupid things, like using my master ball on a skarmory.
Of course, I did that thing where you only ever use your starter, so that was a pretty bad beginners mistake (and I continued to do that right up until DPPt, though my first diamond play through did that to some extent)
Actually, in the first two gens, I never even used any pokemon besides my starter and the first five I caught because I didn't understand how the PC boxes worked
Really, the only reason I got as far as I did in crystal was because my mom helped me (because she was awesome)