This is just a set of advice to keep people from regretting wasting resources they didn't know they're wasting.

Let me get this out of the way first: More stars = stronger. Starting at 4*, girls have passive skills, and passive skills get better after evolution. They also have better active spells, are more likely to have character quests, and get more out of gifts that you give them. 5* are even better, and 6* are even better... overall. 4* is the cutoff for having passive skills, though, so focus on 4* and higher before 2* and 3*.

As for resources:

Exploration points. You can get flower gems out of these, so make sure you use them up before leveling even if your stamina bar is already full. Also keep track of how many you've done or pay attention to your Quests so you don't waste explorations. They do give stamina back, so normally you will want to wait to explore until you need stamina... but it's most important to make sure you use them.

Raid points. Use them whenever you can. Just keep track of your raid quest. Later on, you'll want to save raid points for bosses that are stronger for the better rewards. How do rewards work? The person who finds the raid boss gets a fixed reward, and each person who helps kill it gets a contribution-based reward. What this means is you don't need to worry about having no raid points when a boss appears; just click the "ask for help" button and you'll still get some reward.

Stamina. Assume that you'll end up getting a bonus stage after the next fight and plan accordingly. Save that stamina just in case. If you have a lot of stamina but are relatively close to leveling up, use some special missions - they give low exp to stamina and special rewards.

Gifts. Pick one girl per gift type to give presents to. Ideally, sort your units by rarity and give presents to the rarest first, but the most important part is getting to 100% asap, because you'll get a flower gem each time and rewards every so often from main quest. In order to get the daily quest, give gifts to one girl, claim your quest reward, then do the rest of your gifts. You can get one of each type of gift from a daily quest. Every day there is a special quest that gives gifts as loot. Getting 3* clears in main quest often gives gifts, and you can sometimes find gift items in normal boxes and exploration.

Manyu. Upgrading costs more the higher level the girl gets, so be a little efficient when you can. I separate the levels into three, so the lowest level manyu gets a girl to 20, the middle level manyu gets her to 40, the highest level manyu goes the rest of the way. Within each rank I pick one girl of each color to get the manyu. Personally, I go with the 4* and higher first for various reasons explained below. 2 of each low-level manyu are given out from daily quests each day. Twice a week there are special quests for Manyu. You can get low level manyu from normal gacha. Otherwise, you get random manyu from Exploration and from bonus stages.

Ampies. Permanent stat boosts. This is what makes my disclaimer at the top a lie. Is Mint your favorite character? Pump her full of ampies and eventually she'll be strong enough to go toe-to-toe with 6* units. Unfortunately, Ampies are rare and make only a small difference each time, so my strong recommendation is to pick your favorite girl, whoever she is, and give her all your ampies. There may be a limit to how many ampies you can use per girl, but I haven't seen one personally after 30 of each on one character in JP. Ampies are most common from event rewards, but are also rarely found in treasure chests.

Normal Gacha. Upgrading a girl using the same girl as material gives more accessory slots up to 4. If the girl used as material has the same skill, there's a chance of increasing the skill by one per girl up to level 5. Many 3* girls share skills with 2* girls, and even a 4* girl (Saffron Tulip) shares a skill with a 2* girl... What I recommend doing is Locking girls in the Character Equip/Lock section as you get the first one of each, using duplicates as upgrade fodder only if you can improve skills or equipment slots with them, then sell any leftovers. You'll need to save up gold. In JP there was a daily quest for using gacha points, so it was worthwhile keeping some in reserve, but here there is no quest so use the points as you get them; you might get lucky and get some 4* girls. The best way to get normal gacha is through doing main quest and going after every bonus stage.

Accessories. Ring, bracelet, earring, necklace. You can have up to one of each on each girl depending on how many slots you have unlocked. On the other hand, you can only use 20 girls at a time, so you only need the best 20 of each type of accessory; don't waste flower gems expanding capacity. When you hit level 40 and are able to upgrade accessories... be careful. It's a huge money sink, and the 4* rings and 3* bracelets, plus event accessories, are the items I'd suggest upgrading.

Flower Gems. Premium gacha is a scam! Having said that, luck is what it is. Just make sure you have 50 and get a bonus roll. At the very least it will help you get 4* units. For expanding unit capacity, I like having at least 11 free spaces, so I'll spend flower gems to expand unit space. Restoring raid points and stamina is never worthwhile - unless you're 10 minutes away from getting something from an event of course. By the way, at least in JP there were occasional bonus reward gachas like "you also get some manyu" or "you also get dragons" or "there's a higher chance of getting blue units". So it's a good idea to hold on to flower gems if you're not sure. You can rarely get flower gems from exploration, but the main sources of flower gems are daily login and 3* clears of most missions.

Gold. Such an easily-forgettable resource. Some gets consumed when you upgrade, you get some from each dungeon, but you can ignore it, right? Wrong. To evolve a 5* unit, you need 150k gold. 4* need 100k gold. 3* need 80k gold. 2* "only" need 50k gold each. Equipment uprades are also expensive, and they might possibly do what JP did - add monthly 4* units you can buy for gold at 150k each. It's not hard to earn gold, but once you reach a certain point it will be just as easy to spend it.

Life Crystals. Save them, save them, save them. If you get lucky enough to get a 6* unit, you'll need 500 life crystals for one (guaranteed) skill level increase. Even 5* units require 250 per skill up. Don't be discouraged by the fact that you can only get maybe 4 or 5 a day from daily quests, though; they can be drops in various event dungeons.

P.S. Do the daily quests when you can!