What to buy with 1000 platinum if you are a new player?

I'd say keep it for when you really need it, and you will need some at some point. As others pointed out, inventory slots are the biggest plat consumer, and as you expand your arsenal, you will feel it. Then is catalysts and reactors, which normally aren't available in large quantities - i'd say buy catalysts but wait and be patient with reactors, as they have a much lower consumption rate and you'll eventually get all that you need for free.

You can get a catalyst weekly from duviri now, so idk if its worth spending plat on them.

Honestly I'd hold on to most of it until you get a feel for the game. Spend a bit of it on slots (though you can sell most of the early weapons once you master them, they aren't that good), and once you start getting weapons and frames you like orokin catalysts / orokin reactors. Then, once you know what you hate doing, spend it to avoid doing that thing to farm something.

If you want to splurge on something early, buy the prime version of a frame you like (or like the look of) off of http://warframe.market. Prime frames honestly aren't that much better than the regular versions, but they're tradeable, and being the best version of what they are it's easy to justify investing forma and orokin reactors in them. Note that you'll still have to build the frame, so make sure you have or can get the materials required for the blueprint.

Þe exception to þe rule of ‘prime frames aren’t þat much better’ is frames wiþ exalted weapons. Often þose frames can be significantly better

@byankkito Slots and vaulted prime parts

I'd start with gear slots. Maybe a cosmetic or Octavia's instruments.

A bunch of weapon and warframe slots would probably be most useful. Save the platinum and buy them as you run out of slots. Maybe a nice colour palette for better fashion or deluxe skin if you have already found a frame you really enjoy.

Dont spend it without actual need. Use it gradually for stuff you need like slots.

Slots for weapons and Warframes
A bunch of orokin reactors and catalysts
When the need arises, affinity or resource booster for massive time savings (this is what helped me go ham and ascend ranks fast af)
Resource boosters help you get more plat. How? Void traces or trading open world resources like I did.

Shoulda bought the biggest plat bundle honestly, best value after all. Just stick to buying inventory slots and maybe a deluxe skin bundle for your favorite warframe.

Inventory slots are the single biggest cost in this game, followed by potatoes. Depending on how patient you are, the unvaulting system of Prime Resurgence has a rotation of about a year. Investing two potatoes in a frame and then their prime once they're unvaulted is 40p, which is ultimately cheaper for most frames than the cost of a set + 20p for the potato. The only primes that are actually notable upgrades are Volt due to the extreme increase in energy pool, and to a much lesser extent Nidus and Valkyr due to better low level scaling from increased armour.

In terms of which weapons to keep, hang on to anything with an Incarnon adaptor and anything part of a crafting tree, which you can check here: https://warframe.fandom.com/wiki/Weapons_Required_as_Crafting_Ingredients

Weapons you get in the early game are often not worth keeping and that means they are not worth putting a catalyst in.

Save your plat for slots and for investing in gear you enjoy.

The first weapon you can get that’s worth keeping is probably Dread. Atomos, Cerata, fang prime, also stay strong as you rank up, as do the rest of Stalker’s weapons.

buy glaive prime and mostly keep it for the ongoing maintenance - orokin reactors/catalysts, slots, maybe some annoying mods that are nice that'd be my advice

Double it and give it to the next Arcane Energize Seller.

That was a half-joke; you could opt to save it and wait until you get around 2,500 plat and get a maxed Arcane Energize. Otherwise, I recommend buying some meta prime weapons and/or frames if you want to spend it immediately.

As another comment said, use it gradually on what you want.

One of the most useful things are slots, warframe slots and weapon slots.

However don't spend everything right away. You will keep the 1k plat, and you can also trade with other people if they have parts/mods you need (warframe.market) (don't get scammed tho).

The necramech grind is kinda a pain in the ass. Don't buy it from in-game market, but at some point buy the parts from warframe.market. other than that slots until you have good enough weapons to consider upgrading with plat.

I'd get a bunch of cosmetics and save a bunch for Warframe and weapon slots.

Being able to customize your gear makes it a lot more fun to use IMO. Since you can farm for functional items like Warframes, weapons, mods, etc., I find it nicer to just follow the progression of the game.

I recommend checking out Color Pack: Alpha, Color Pack: Beta, (maybe the Kuva Lich Hunter Collection), the Shi armor set, the Sigma Series armor set + syandana. You can also just have a look up armor sets and syandanas in the Market to see what's available.