Moon scolls: cave keys should always be a priority, save up at least 1050 for cave key rebates, which comes roughly once a month. Spend the rest on medium refine runes until you finish them. You c*so consider buying the training pots in there, they will help you gain power fast, but they're also a bit overpriced, so lower priority for now

Sun scrolls: refines

Coupons: save them up until you can complete a full fuku rebate (which is usually around 35k), don't spend any of them before that

Ninjas: your first target should be to get a good focus ninja: if you're free to play or low spender, then ETR Madara is the best one you should aim for. If you're very new you already got him 3 stars as part of the starter pack, but to make him actually good you have to upgrade him to 4 stars and complete his skillbreaks. That's what your coupons will go for.

General rule is that every resource that has rebates (cave keys, charms, refines, seal scrolls, summon scrolls, coupons, battle barmor threads) should be saved up and spent during their rebates.