This page of The Witcher 3: Wild Hunt guide contains a walkthrough of the game's prologue taking place in the Kaer Morhen fortress. This walkthrough tells you, among other things, what activities precede the proper start of the game, describes the meeting with Yennefer, and how to correctly complete the training with Vesemir and whether it is obligatory.
Selecting the difficulty level and transferring the choices from The Witcher 2
Before starting the proper game, you must choose difficulty level and decide whether you want to see hints. Normal difficulty level is the optimal choice for completing the game for the first time. Fortunately, the game allows you to change the difficulty level from the options menu without having to restart the entire campaign.
Additionally, you must decide whether you want your decisions to be transferred from the second installment of the series. If you have a PC version of the game and saves from Witcher 2, you can upload them. If you don't have saves or if you're playing console version, then you can decide whether you want to determine most important decisions - Imperial Audience main quest - or does the game should make those decisions for you.
The Witcher 3 Wild Hunt does not have a character creator, and this is because the main character is always Geralt. Fortunately, during the course of the game, we can freely modify the appearance of the hero's equipment and gear, as well as choose different hairstyles for Geralt.
Prologue of Witcher 3 takes place in Kaer Morhen fortress. It is a tutorial that allows you to learn the basics of the game. You start in Yennefer's bedroom. Use Witcher Senses to find a key to the bedroom door - it is lying on one of tables, on an open book. Walk down to the lower floor of the fortress to meet Vesemir and Ciri. You can agree to participate in a race to the lower courtyard or refuse. If you choose the first option, then you will learn the basics of moving - sprint, climbing, jumping over obstacles and jumping down.
On the lower courtyard Vesemir will ask if you want to participate in a sparring. You can tell him to remind you the basics or immediately start the training. If you picked the first option you will learn all of the most important fighting techniques - choosing the right sword, marking targets, fast attack, strong attack, dodging, counterattacking, using signs, throwing bombs. Soon the prologue will end.