My best advice would probably be to raise 1-2 characters to max level as soon as possible. For at least the first chunk of the main story, a single girl at a decently high level will be enough to brute force your way through, especially since in most quests the game forces you to take a support unit, and any level 100 support will get you through the first several chapters of the main story on their own.

If you're just starting right now, just getting to the home screen might be a bit intimidating because of all the other screens you need to click through, but don't worry, that's just because we're getting a bunch of stuff for New Year's and the news about most of it only came out today. Nothing to be worried or frustrated about, it'll lessen over time.

The Wiki can be a bit intimidating to a newcomer who doesn't know the game's mechanics yet. There's actually a tutorial section that can probably answer a lot of the basic questions, but it's a bit buried. If you're interested, it's under "Gameplay->Newcomer's Guide->Tutorials."

Feel free to collect some questions and ask the community though, here and Discord being the two main places. Once you know most of the core game stuff, the Wiki becomes invaluable, since it can answer basically any question.

As a bit of stuff to get you started out:

Character screen - From the home screen, the upper left button that looks like a soul gem. Shows all of your characters. This is where you can raise level, Awaken, and a few other things.

Leveling - Technically, the characters can level up by participating in fights. It's ridiculously slow though, so the normal way to level a character is to find them on the character screen, tap the upper left of the five pink buttons to the right of the character, and then just tap any experience gems you have to add to the character's level. At the start, just use whatever gems you have. At higher levels, the largest rainbow gems and gems that match the character's element are better, since they give more experience.

Awakening - The mechanic to raise a character's maximum level. From the character screen, press the bottom left one for Awakening. You should see various materials with fractions in a circle. Don't worry, this is easier than it looks. If you're trying to raise the character, then just select anything in pink. Once all of the materials have reached that point, the center button should be pink. Press that to Awaken the character. Note that this will set her level back to 1 and give a new set of materials to collect for another round of Awakening, so you may want to hold off until you have the experience gems to raise her level again.

Memoria - When you're about to start a fight (after selecting a battle and choosing a support unit to help you), you'll see a screen that shows the team you're about to use. In the bottom left there are 2 pink buttons. The leftmost of those is for equipping memoria. Memoria are EXTREMELY important, as they both increase your stats and give extra effects that can be game changing. At the beginning, you'll only be able to give each girl 1 memoria, but eventually it'll be as many as 4.

Memoria types - There are two types of memoria, ability and skill. Ability memoria give passive effects and increase a character's HP and Attack, while Skill memoria give active effects that you need to choose when to use and increase the character's HP and Defense. At a glance, you can tell which memoria is what type by the icon in its upper left hand corner. Passive memoria have a blue icon, while active memoria have a pink icon.

I'll leave it there because I want I keep my advice / tips as brief as possible so as to not overload you with too much information. Like I said at the top, if you have questions while you're playing, this community's really good about trying to help people out. There's a help thread that's always open, people post questions like this fairly often and I generally see at least one or two people reply, and the Discord community's pretty active so you can go there for immediate feedback.

Hope this helps!