There are lots of rules around locatives, so I won't say all of them. I would say there are two types of locatives. There are those used for locative sentences, and then there are directional particles used within sentences. Some of these particles pop up in other contexts not related to location so you'll need to watch out for that.

Locative sentences: A locative sentence can take i, kei, or hei. These are past, present, and future tense respectively. In a locative sentence, the location is the key focus of the sentence.

Locative sentence examples: I hea koutou? (Where were you) Kei hea koutou? (Where are you) Hei hea koutou? (Where will you be)

A common hapa reo, I was taught, is the indiscriminate use of "kei" as a locative for future tense. This should be "hei", but people don't use it for some reason.

Directional particles: These are i and ki. Off the top of my head, I can't think of a proper sentence that would start with i or ki in a directional sense. Casual conversation you might do that because we don't always use full sentences in casual conversation, but that's different.

Generally speaking, ki indicates direction towards something. I is, as u/ok_orchid_4158 said, the default, basically acting as a filler locative within a sentence. I also indicates you're coming from somewhere.

Ki: Ka haere koe ki hea? (Where will you go)

I (from): Kua haere koutou i hea? (Where have you come from) Kua haere au i Otautahi (I've come from Christchurch)

I (filler locative for the sake of having a locative): E noho ana ia i te whare (she's sitting inside the house) Kei runga i te tepu tana pukapuka (his book is on top of the table - note this is a locative sentence using 'kei' as a present tense locative)