IMO you’re better off without guides. They usually steer you in a pretty specific direction and often focus quite a lot on ‘efficiency’, which might not be the most fun experience for you.
The most important thing is setting goals for yourself. It gives you a solid point to work towards, which helps with motivation, and allows you to determine which parts of the game you can ignore for now, which can help you feel less overwhelmed.
A good goal could be unlocking X planet, farming Y Warframe, completing Z quest, etc. As long as it’s interesting to you, it’s good.
As for learning how stuff works, you should be able to grasp most of the important stuff by just interacting with systems as they become relevant. If you’re not sure how something works, you can ask in the in-game Q&A Chat.
There’s a % chance that the G3 will appear in the first few minutes of any Grineer mission. So, just do a bunch of short-ish missions and they’ll appear eventually. IMO it’s not really worth actively trying to get them to show up.