Simple version:
The gameplay loop is do some missions, check your foundry what you can build, do some more missions.
While doing this, you probably unlock some nodes, earn credits, endo and resources.
More detailed version:
After knowing exactly what you want to build and need very specific resources that you haven't gotten over time, you want to search on wiki where to best farm the resource and repeat a few of those missions.
Very detailed version:
At first, most of your progression is trying to max things out. While warframes themselves are quite limited (you only have a few) and should max out by themselves way in time before you earn a new one just by playing, most of the mastery points is earned by trying out many many weapons.
I went with the fast lane and basically fought with low level easy to make weapons for a long time (all the weapons that are purchasable with credits, or weapons build with blueprints that cost almost no significant resources). For in case you didn't know yet: you can check any weapon that can be purchased with platinum, but instead of purchasing it with platinum, you can buy the blueprint instead with credits. Those blueprints require certain resources to build the weapon in the foundry and that's where your resource farming goals come in. Thus you have the loop of "want to train a new weapon" > "check what weapon is cool" > "farm for resources for that weapon" > "build that weapon" > "play with weapon" > "repeat".
If you have trouble with content, I suggest finding a reliable weapon to fall back on, so you can train all your new stuff but if things get rough, just blast things away with your equipped favorite weapon. In my case that was the Hek.
Do actually make your warframe and weapons strong however, is not so much about the mastery level, but rather the mods you put in it. So a level 30 anything, is actually not that strong, it's just the starting point. For starters, the best things to put in your warframe is extra vitality. The default vitality mod gives a minimal boost and you should upgrade the rank of that mod as far as you can (before the endo price gets too expensive at your stage). The same goes for weapons: usually the first mod you want to upgrade to actually feel stronger are the damage mods (the ones with +% damage). The difference between a single upgrade damage mod vs a bunch of random unranked mods in a weapon is huge.
Future info:
After you've gotten used to the loop, you might think that's about it. However, the loop only gets more emphasized once you start using Orokin Reactors (for warframes) or Orokin Catalysts (for weapons) and Formas (polarisation). The first 2 (also known as potatoes in the community), double the mod capacity, but it's a very rare item that you initially started with a free one of each and can be earned through some alerts. This is what makes a "main" a "main". My Hek is potatoed. Formas are a way to add more symbols to a warframe or weapon. The symbols you see on warframes and weapons basically mean that if you put a mod with the same symbol in it, it costs less mod capacity. If you put a mismatching mod, it costs more mod capacity than normal. Thus if you invest a lot and keep retraining (you can only use a forma on a max warframe/weapon, which resets it back to unranked and you have to relevel it, earning no extra overall mastery points), you can create a warframe/weapon that holds a lot of strong upgraded mods and become really strong.
I believe the Trinity Prime comes potatoed by default, so if you want to hold back on your first potatoes, you might wanna use that frame. In my case, I just potatoed my starter frame Mag since I used her so much in the beginning before I started farming other frames.
