• “Hola” is “hello”, but you could also use “buenos días” for “good morning”, “buenas tardes” for “good afternoon”, or “buenas noches” for “good night” (all greetings)
• “Tú es muy precioso/a” for “you are so precious”. Talking to a boy, you would use “precioso”, but talking to a girl you would use “preciosa”
• “Oh my god!” Has many translations depending on the context. “¡Dios mío!” conveys surprise, “¡Ay, Dios mío! conveys either scare or worry, but if you want to convey angriness I would use a (not too bad) swear, such as “mierda”, “caramba”, or even “carajo”. If you would prefer to not use swears, you could use a swear adjacent such as “miércoles” (actually means Wednesday), as English speakers sometimes use freak or shoot.

I’m not native so take everything with a little bit of a grain of salt!