Choosing a knife is a very personal matter and there are no fail-safe rules.
Initially, I would advise you to watch the video in which I explain the use of some basic knives.
After that you can buy a good kitchen knife, not particularly expensive, but not too cheap either. ‘Weigh’ the knife in your hand, see how it fits and, once you’ve done this with several knives, decide by following your instinct. You must begin to work it every day so as to familiarize yourself with it and assess it.
Finally, an essential matter, almost as important as the purchase, is the sharpening of the knife. Any knife, as good as it is, will stop being effective if it is not sharpened correctly. Don’t forget: a sharp knife is a safe knife.