i want to read a horror manga/ watch horror anime

Uzumaki! that one is terrifying.

Watch Higurashi, you'll love it.

One I recommend is Another. It's on Crunchyroll. It has a lot of gore so beware.

You mentioned manga in the title so I'll shill for Mieruko-chan, anime adaptation was announced not too long ago but isn't out yet.

castlevania (like van helsing in cartoon form). also uzumaki is a good horror. death note. tokyo ghoul.