This is a good one to start with. Initium is a beginning, the initial stage of something. A or ab is a preposition meaning from or out of. So ab initio means from the beginning.
It is used mainly in legal or scientific contexts. It can also be applied to a piece of literature. If a story gives us the events from the very beginning, it starts ab initio. Otherwise, it starts in medias res.
Ab takes the ablative, which means it is always followed by a noun in the ablative case. It’s right there in the name, so it’s unusually easy to remember. If you have any doubts, remember ab initio. Initium is a second declension neuter noun. Initio is the ablative singular.
Ab ovo has a similar meaning. Again that -o ending suggests the ablative form of ovum, or egg.