This に is a particle that shows where someone or something goes to, or where it is. When you move toward a place, you put に after that place: 学校に行く means 'go to school'. It also marks the place where something or someone exists, so 公園にいる means 'be in the park'. In addition, に can indicate a specific time when an action happens, like 7時に起きる (get up at 7). It is one of the most common particles in Japanese, so you will see it everywhere.

Be careful: に is also used to mean 'and' or 'in addition to', as in 犬に猫 (dogs and cats). That is a completely different usage. Here we are only talking about the location/destination meaning. When you see に after a place or time word, it usually means 'to', 'in', 'at', or 'on'.

In speech, に is used all the time, both in casual and polite conversation. It is not formal or literary. Just remember that for existence verbs like いる and ある, the location always takes に, not で.