Most people should have their friends code under their avatar.
As for advice, learn the game's systems, use a bunch of different support Servants so that you know what they can all do instead of just relying on the one that always onw-shots everything, and don't listen to the meta, unless they say a character's just flat out garbage.
Oh, and never use your quartz in the story gacha, unless you really are fine with just getting random tat.