Since you are a beginner in terms of how much you have progressed, I suggest having warframe and weapon slots as your top priority, they aren't that expensive and the only way to get them is with platinum. You could maybe buy a skin or armor if you have a bunch to spare to give yourself a treat.
Another thing that's important is the things you shouldn't spend platinum on, like:
-Buying weapons/warframes from the in-game market, you can farm them easily by playing, if you don't play a lot you could buy one but if you play a bunch you'll regret it later.
-Rushing things from the foundry, it's just a day for most things to craft, unless you really want something.
1000 plat is a pretty good amount, you can get a bunch of things with that, have fun shopping :)