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

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.

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.

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).