The water from my bathroom tap comes out at an angle (as though I had my finger over the stream). The direction of the water changes as I turn the tap. It also makes squealing noises and sometimes sputters, making loud knocking noises when it does.

It behaves better if I run the shower at the same time. If I tighten it shut really hard, sometimes it will return to normal for a bit. I mostly stopped doing that though cos the seal broke and had to be replaced.

What's wrong with it and how hard is it to fix? I have zero plumbing experience.