A real shitty Spanish lesson
Warning: This post contains strong language reader discretion is advised.
The verb is cagar. Cargar should not be confused with the similar-sounding word cargar. The word may sound like cagar if you mispronounce it, and you may receive strange looks. "Cagar" means to screw things up in colloquial terms. A milder form of the word in this context is meter la pata.
Spaniards, for some reason, like to shit on everything figuratively speaking in anger. Two particularly stand out to me. The first one is Me cago en la leche ( I shit on the milk). That made me laugh since the inventor of it must despise milk for some reason. To me, the most shocking one is Me cago en Dios (!) I shit on God. There are quite a few godless people out there who'd love to do that. It might have originated out of hatred for the Catholic church. The euphemism Me cago en diez is funny in both senses. It's kind of crazy that they went from slandering God to insulting a number that represents perfection.
Mierda is the noun in all cases. Except when used to describe low quality, it is not a metaphorical word, like its English counterpart. If you're mad at the lemon you bought, you could sayCompré un carro/coche de mierda. I bought a piece of shit/shitty car.
What if you want to call a person a piece of shit? You can use de mierda as mentioned above. To sound Venezuelan you can say mojón as s in the video made by Venezuelan creator Kevin Monslave aka Gamerkevmo Sé tú mismo siempre que no seas un Mojón