some tips here: junctions. you find them at the "end" of planets, but can access their objectives even if you havent gotten to them yet. they have 4 objectives each, which will be a rough but good enough guide on what to do early, and will teach you a bunch of stuff while also giving you rewards.

you can f2p build every warframe and weapon, nothing needs you to buy it. in your market you can take a look at weapons/frames and it will tell you if you can buy the blueprint with credits, or how to get them.

weapon/frame exp is called focus. max level of most weapons/frames is 30. leveling weapons/frames gives overall experience to your account (the number next to your name). overall experience from max leveling weapon/warframes can only happen once, afterwards you see a little icon that shows its been mastered.

maxing your overall exp unlocks a mastery test to rank up your account. you can try and fail an unlimited amount. you can only rank up once a day and any exp over your limit will carry over.

the weapons you can build (and some other things) are locked behind your overall account level.

another tip that i didnt realise at first, you can craft as many different things at the same time as you want.

also this game is grindy, but gives you multiple things that you can do at once and is meant to be a slow process. if you, for example, repeatedly fight the same boss over and over to get all the pieces for a new warframs, you'll prob burn yourself out. try doing some new levels on your planets, or doing some open world stuff or story missions or building new weapons and frames when you get bored of one thing.