For newbies

Play through the tutorial, of course. 

But as a starter unit, I suggest Selena. Like scroll to the blue female. You can ignore what is written there first and click proceed or something. I will explain much later in many posts later about why.

Then, first keep on questing. Summon when you can. Until you have at least like 3 units from the summon gate. RNG is involved here. It is totally random(hopefully). But sometimes, there are rates ups. Basically, there is a higher chance for certain units to appear in the summon gate. I suggest that after you have 3 units, save up for this batch of units. Avant, Chrome and so on. Their names may be a bit different, but basically a guy in black suit holding two scythes is Chrome. He will come soon. 

I will assume that you know what each stat does. If not, fine.

HP basically more health points

ATK more attack damage

DEF less damage will be taken for your units as compared to units with lower DEF

REC basically how much heart crystals will heal for that unit and how much healers will heal for the entire squad.

Now, I will explain types.

You know how underneath the level of the unit, there is a type? It is either Anima, Breaker, Guardian, Oracle or Lord.

Anima will give you more HP and less REC

Breaker will give you more ATK but less DEF

Guardian will give you more DEF but less REC

Oracle will give you more REC but less DEF

Lord will give you balanced stats.

Most people will feel that Anima is best for everything since REC doesn't matter too much if you have a healer and having more HP will help you survive more attacks. However, for arena, it is better to have breaker units. So that you can deal more damage to your opponent and hopefully kill them before they do. The concept of arena is simple. Nuke them before they do. Arena can take time when you are new but they will take way less time and way simpler the higher level will go. 

I will leave the rest for you guys to figure out. Like, what fun is in the game if everything is explained by me?

Post here on what units you get and I will comment on it. But if I really don't know, just use it.