It's like most other RPGs in that you WILL be overwhelmed at first. You wouldn't understand (or forget very quickly) how such and such stuff work. That's perfectly normal.
My advice is just run along with it. At first, just focus on how to slash and dodge. That's quite enough for normal difficulty.
Overtime you will be naturally curious as to how to improve efficacy of your combat. Then you will learn how to use signs, seriously look into skill trees, and swap your gears with what you have in your inventory. Lastly you will learn to boost your stats via alchemy.
In no time you will be a master witcher before you know it. But just one thing at a time if you're so overwhelmed.