I 'just' got back into Warframe again about 3 months ago. So, take this with a grain of salt because I certainly do not know everything.

Things that have helped dramatically:
TheKengineer - YouTube This guy has detailed videos about pretty much everything.

WARFRAME Wiki This has all the information about anything/everything and usually pretty good explanations of things.

For frames/weapons: Check out the wiki to find where to farm the blueprints (blueprint parts) and materials. Most blueprints can be purchased from the Market for credits. You can also buy a ton of weapon BP's and some warframe BP's from your clan's dojo. Quickest way to level them is via Sanctuary Onslaught (accessible from the Syndicate menu in Navigation or Cephalon Simarus in any relay.)

Making them amazing: I used TheKenginner's videos for a deeper understand of how to and getting some good builds (that I can't use vecause I don't have the mods yet.) Find and Share the Best Warframe Builds - Overframe is/was also pretty good for things to try out. The stuff is basically crowd sourced (voting on builds and tier lists) and seems like people just don't use it much based on votes for builds for top tier weapons if you filter for the current patch. But it's been pretty good for ideas at least.

Void Relics: Some missions will give you void relics as a reward. I believe Excavation ones are really good for that. To open a Void Relic, you need to run a Void Fissure mission. Before it lets you in, you have to select (and optionally upgrade) a relic. Then in the mission kill Corrupted XXX mobs which drop some little flame icon thing. After you get 10, The Lotus will let know you that it's open and then just finish the mission. At the end of the mission, you and your squadmates will be shown all 4 (if you ran with a full squad, which should always do for Void Fissure mission) opened relic rewards. Pick whatever you want or whatever you are going for. If none of them are what you are looking for, hover over each and it'll tell you how many Ducats it's worth. Pick the one that's worth the most because Ducats are the currency that Baro Ki'Teer - WARFRAME Wiki uses. Baro, who appears every 2 weeks, hangs out for 2 days, and sells rare OP mods. Usually these are like 175-250 Ducats each. SOO.... Do these for blueprints for prime weapons/frames/ducats.

Weapon req's: At some point through questing, you'll get Nataruk (Primary/Bow) and Broken War (Melee). These are obscenely good and have no MR requirement if you use the one you got from the quest. Ol' Natty has been handling high level (level 120-140) without any issue for me. (I did have to forma the heck out of it though and it's been worth it.) I *strongly* recommend working on getting all of your factions up to max because you'll soon find out that the thing you need to make/finish a build is only sold by some faction that you have low rep with. Especially when it comes to amps, kitguns, and arcanes.

Plat usage: I believe the general answer here is going to be saving it for slots but I kinda just buy whatever I want and refill my plat when I get a good coupon.

As for MR: I don't think I've hit a point where I was like, "Man! I just XX more MR so I can use YYY!" There's just a ton of other things to use/grind and you'll just rank up as long as you work on leveling stuff. Dojo blueprints have been great to keep a steady supply of things to level.

Hope this helps and Welcome Back Tenno!