Good thing on noting the beginning platinum should definitely be spent on slots. I was tempted to get a cape (syandana) but my sensei told me no.
As for using a potato (nickname for orokin reactor / catalyst), even if you do it's not the end of the world. I potato'd my Mag and potato'd my Valkyr, only to sell (delete) them later for their Prime versions. But since those are vaulted frames, only much later I replaced them (vaulted frames are not normally obtainable until they are unvaulted and put back into the rotation for a short period). Easily spend like 100+ hours on my Mag and 300+ hours on my Valkyr and enjoyed those times very much. By the time they were replaced I had enough new potatoes.
Having a frame you can call your own is the best feeling in this game. The one frame you keep playing and master it's playstyle. But like Weltall Zero mentioned: in the beginning you'll be playing with Mag for a while. Each planet has a boss to farm a non-Prime frame for, try to find the one you like (I recommend watching videos and reading the wiki on it's abilities).
Weapons though, keep trying as much as possible. Especially the ones you can buy with credits (without farming resources) are easy tests and can be sold to make room for more slots (generally anything hard to come by, especially one time event weapons, you try to keep).
The most common problem is finding out why the game is fun, especially because the beginning seems so easy (super low level enemies) and enemies seem to repeat themselves. I can tell you this: the game will get harder, enemies get tougher and your goal is to match them, or surpass them. You'll hit a breaking point when you don't just use what you got, but try to farm what you want so you can customize. Instead of balanced builds (a little bit of health, a little bit with strength, a little bit of energy, a little bit of damage), you'll start sacrificing stuff to become an AoE monster, or a tank god, or a melee blender, etc. This game isn't as fast with communicating this power fantasy as other games, but it's there. But don't worry, you have enough challenge with your first main goal: trying to unlock all planets (and has some good high notes by itself).
As for this power fantasy, keep in mind that for mods you generally want to upgrade the main general ones. I remember putting a bunch of unranked mods on my stuff at first. But for starters, it's better just to upgrade 1 vitality as much as possible for warframes, and +% damage for weapons. Mods are upgraded once and you can re-equip on other frames/weapons, so those general ones are pretty much always useful (until maybe much later you get like primed versions or have specialized builds).
You'll start experiencing most stuff naturally by just proceeding with the main line story, unlocking planet to planet. Mag happens to be really good at taking out junctions (the connection to the next planet) with her cheesy bubble :)
Wiki is your best friend to search where to find materials for crafting. Some materials are locked on further planets, some can be crafted right away. In the beginning you might be doing this quite a few times and farm specifically for materials (resources). But the general rule is, there's no penalty for just playing the game, you'll always stock up useful resources. That's what I think is the strongest point of the game: you always feel like you're making progress. I still visit Earth plenty of times.
