So in answer to you not knowing what moves to use and what they do. There are lots of guides online about what moves to use as well as in-game if you go to your pokemon roster and click on a pokemon. Go to their moves and hover over the move, this should give you a summary of what that move does as well base power, secondary effects and accuracy. As a beginner that is the best thing to do and not focus on anything else since you're still trying to get a feel for the style of the game and its general mechanics.
Edit: I'd also maybe look up what Pokemon are, in general, special or physical attackers after you catch them so you know what kind of attacks are better suited for it.
As for the items, I'd again say that since you are a beginner not to focus too much on the items. I would recommend only focusing on the potions, revives, elixirs/ethers and Exp Share. and of course the Z Crystals since that is the whole point of this game. After that though, I would just not worry about the other ones since majority of those are for competitive use. As a beginner its better to just understand the basic mechanics and ideas rather than try to learn everything at once. That'd be a major information overload.
As you play more, you can begin adding more strategies to your playing style such as other items, more competitive movesets, mega evolution, breeding, evs and ivs.
For now though it'll be a lot of trial and error to get used to the game and figure out what does and doesn't work. And it will be frustrating but of course if you have any questions at all, we have plenty of people here ready to answer any questions you have.
On a side note, any questions about the storyline and where to go and such, I'm happy to assist with.
