This is a story-driven, not so safe for work, shelter management, "gacha" game. As such you will encounter many characters and have a wide variety of resources to utilise in building and upgrading your shelter as well as the equipment your girls will use to fight. In this this guide, I will try to give some basic tips in order to help you get through the game easier.
Your immediate goal is to play and get a feel of the way the game works. Doing the tutorial and simply playing, you will beat up to mission 1-6, probably be at shelter 4 or 5. If you try 1-7, you will likely get shredded as the enemies get a power spike. Here you will want to constantly be running 20 minute expeditions, (40 minutes can be fine if your girl has 100+ power). Gather equipment, gather materials, have your facilities be producing, make sure you have someone in the radio station to pull in workers to staff some facilities as the champions wont be enough to cover everything. Level Trixie, Andrea, and XX to 5 as they're mandatory for the next few missions which can be hard. Try to synergize some equipment for them. By that I mean, if you have a weapon that scales off agility, give that to one of the three girls and then give them armors and a trinket that boosts agility. Your biggest priority is whatever your shelter tasks are. You can also look here to see what the next Shelter Level will require to help you plan ahead.
Eventually you will have enough materials to get all their weapons to at least level 16 and three starter girls will be level 4 or 5. If you can boost Andrea's trust level, that helps a lot too as her heal gets doubled at trust level 2. This will take some time, probably one day of really active playing or 2 to 3 days of casual playing. Once you have about 600 power between the three girls, you can punch it through a few more levels and unlock a few more champions. You'll then be gated at Shelter Level 6 as the level 7 requirements can take some time. Use this time to run expeditions and look over your equipment. Check what stats facilities scale off of and try to stack those stats on girls you have working them.
You'll probably want the Server Room (which produces the boxing glove training materials) prioritized. After that send some girls of lower power to the 20 minute expeditions. The recommended 25 power level there is insignificant. Shove whatever people you have wherever they're best in in the remaining rooms (you'll probably wanna check which girls are good in the Forge as the enhancement can take anywhere from a couple hours to a couple days depending on the girls' stats). The "Drums of War" mission for the Shelter Level 6 task is the seventh mission in Chapter 2 so you'll be here for a bit learning the game, collecting equipment and upgrading. After this, things kind of open up and you're pretty free to play as you want. You shouldn't have too much trouble with things.
Obviously, you're not going to have a lot to work with in the beginning. But as you collect yourself some equipment, make sure to have sets that synergize. If your weapon scales off Agility, make sure whoever is using that weapon has armor and a trinket that also boosts agility. It'll be difficult to synergize equipment in the beginning but as you collect a ton of them over time, you will have more options and ability to do so.
Beyond boosting damage, you will likely want some Intelligence gear to speed up the ELDI Factory and Treasury, Fortitude gear to speed up the Server Room and Utility gear to speed up the Armory and Forge.