New player, what to use mana prisms on?

If you are very new, spend it on tickets and gold exp cards. Eventually buy limited craft essences and fou cards.

Prioritise getting summoning tickets.

People tend to save for shop CEs as they're usually useful for farming in the long term. It costs 1000 MP per copy, so you need 5000 to MLB.

People are currently saving for Chaldea Lunchtime. It gives 10% bond bonus which is valuable for bond farming and is one of the best shop CEs in JP right now.

Mona Lisa CE gives a 10% QP bonus at MLB and is arguably one of the best shop CEs in-game. It's no longer available but will return. However, you need to unlock it with rare prisms and then you can proceed to buy them with MP as usual.

Once you have a sustainable supply of MP (through ember farming or events), the priority is usually: Summon tickets>Fous>EXP Cards. You don't need to immediately spend it all, as saving for CEs is usually better. But always try to get at least all the summoning tickets.