This. 1000 times this.
If I would make a strong recommendation while starting off I would highly recommend investing in:
1. Balls out DPS character (Diluc, Klee, Childe, Xiangling w/crescent pike, etc). In my set up my only level 90 (and probably only level 90 for the next several months) is Klee, my primary DPS character.
2. A healer (I personally prefer Qiqi over anything but lots of decent healers work, but pick one you can do and go with it)
3. A Good support to support your DPS (You don't need to level up your support all the way, but I would make an exception with a Fischl, she's just awesome no matter what)
4. Other support to support your DPS through buffs. Bennett for fire, basically stuff like that.
Be careful using Barbara as a healer, while she's a good character, her water based heal sucks lots of times when you can get frozen. Four star character are perfectly viable, though Amber, Lisa, and Kaeya are probably the weakest ones, followed by Noelle. The "best" weapons for some characters are 5 star, but 4 star are perfectly viable, and in some cases 4 stars are BIS (eg: Stringless bow for Fischl), and even 3 stars (Thrilling tales of dragon slayers book for Barbara). Don't feel like you're going to be gimped by not having 5 star all stars. The only 5 star weapons that make a big difference are going to be for your DPS.
Ningguang is also an excellent Geo DPS. If you build her around DPS, she has some of the best burst DPS in the entire game.
To get Condensed Resin you need to get Liyue reputation to 3 I think, maybe it was 4.