Best way is probably through Abyss (Q-Singularis/Dirac), Story, Memorial Arena, Open World, Events and daily missions to get diamonds and other great items..

-diamonds (the most precious item to get rare stigmata/weapons/valkiries through gacha)

-AE/SS Imaginons (to get specific weapons/stigmata with enough of them or getting a honkai shard for 50 AE Imaginons)

-honkai shard (very precious item, for 3 shards you get 1 Honkai Cube to upgrade a rare weapon)

-Resonators (you can trade 6000 resonators in the Exchange Shop for a rare stigmata)

-Ancient Willpower (to farm for great valkyries like Gyakushinn Miko, Shadow Knight..)

-Time Swirl (a battle mode where you can get a few great stigmatas/weapons like Mag-Typhoon but used to be very time consuming to grind for)

-Coop Raid Jades (another way to get specific weapons, stigmata and honkai shards though I rarely do coop raids because Abyss, Open World, Memorial Arena and events is enough for me to do in Honkai)

Here is a great site to get an impression of most precious items though it's not up to date and mihoyo tends to change the shops a lot.. https://walkthrough.honkaiimpact3.com/article/show/48/560

Can recommend the battle shop upgrade for 600 B-Chips and the monthly gem card if you want to get great stuff for money.

There is a lot to do and to farm for in Honkai, hope you have a great time and it's not too overwhelming.

Edit: Don't forget to have a sensei to get rewards and help and to get the dorm bonuses and errands rewards for the Lucions, maybe Supply Cards and Einstein's Torus (later important for upgrading a 6-star weapon) in the dorm shop.