If you are just starting the game we will soon have all information about it.
Once you get into the game (we are assuming your new) you have to summon a character. To equip the character, click on Make Team and this then click View on the character you want to equip. Then hit equip and choose which slot you want to equip them to.
You will have 3 slots to equip 3 characters.
Then you can play the first raid, either publicly or privately.
This is the link to the game discord: https://discord.com/invite/xuwZEDxYaB