THINK OF YOUR KNIFE AS AN EXTENSION OF YOUR ARM. A REALLY POINTY AND DANGEROUS EXTENSION.
Don’t get loads of them - getting used to one length means you’re less likely to put your eye out or accidentally stab your loved ones. Getting something about the length of your lower arm can be a good start, but this is down to preference.
Anything that gives you a comfortable angle for chopping and slicing is good.
It’s a bit counterintuitive, but the sharper your knife is, the less likely you are to cut yourself with it. This is because you use more force to cut with blunt knife and because blunt knives are more likely to slip. Keep your knives sharp by keeping them clean, keeping them out of the cutlery drawer, and never cutting on a glass or metal surface.