1, "A witcher can forget to eat, to drink, to breathe, even, but a witcher never, ever forgets to care for his blade." Yup, Vesemir was right :) But also, never forget about your armor set, buy and use repair tools as much as you can. Or if you don't have any at the moment, a blacksmith can repair it for you. It's not too costly.

2, At the very begining, you must decide which Path you follow: Way of the Fight, Way of the Sign, Way of Alchemy. Of course you can mix them together. But I recommend following one or two maximum. It's better to specialize. (I chose Signs and I didn't regret it. There are only 2 new tricks with swords, but 5 with Signs, otherwise the other way is stronger in damage.)

3, Try to find and use many different oils, bombs, mutagens, potions, decoctions and crossbow bolts. They will make your life easier ;) Explore the world, not just to discover the mountains, swamps, plains, forests, ruins, seas, etc but to gather diagrams and components for crafting.
Before you start your battle, prepare what you wanna use. A separate piece of armor might have better statistics, but it's better to use a complete set, because they are much more effective altogether.

4, Learn the Bestiary. There are plenty of enemy types in this game, so if you are not a speedruner, you will know their weaknesses. If not, you can check it anytime :D

5, Before you start the fight, check the area: big open place or a small corridor? How many enemies do you have? Where are they positioned?

6, Use your tricks (counter, parry, dodges), I'm sure they will save your life many-many times. And sometimes the best tactics is to keep moving.

7, And maybe the most important thing: if you try to rush your enemy down, you will fall almost everytime, never forget this.

+1: Play on Death March difficulty. No kidding. "Double the pain, double the fun!"

Good luck on the Path :)