You could notice in the texts that sometimes there is a particle не, sometimes there is ни. While talk they both sound like ни, some poorly educated Russians don’t know the difference and write ни instead of не which is inappropriate for a Russian because it’s a very easy thing to remember.
НЕ stands for not. It always goes before the word like in English: Я не занимаюсь спортом = I don’t do sport. Он не студент, он учится в школе = He is not a college student, he studies at school. (студент in Russian is someone who studies in a university). Я чувствую себянеочень хорошо сегодня = I don’t feel very good today.
With adjectives не in most phrases becomes a prefix, except for the sentences with clear antithesis: Анна пришлане в белом, а в чёрном платье = Anna came not in a white, but in a black dress. Without antithesis, it would be something like: Мы живём в небольшом доме = We live in a small house. (though небольшой is somewhere in between big and small).
НИ is only used in ни…ни construction, which is the same as “neither…nor”: Они не едят ни свинину, ни говядину = They eat neither pork nor beef. As it’s a negative construction, we still need не before the verb to show that it’s negative. A positive sentence would be и.., и (with a comma) without не: Она слушает и современную, и классическую музыку = She listens to both contemporary and classical music.
So ни always goes in pair with a second ни, except for probably a word combination “ни разу” that is the synonym for никогда - never. Ты ни разу не катался на сёрфе = You’ve never tried surfing