Must Try... will be confusing and you might even stop playing after 3-4 hours... but you really need to play it more to understand it. If confused check warframe wiki , check Youtube guides , Ask people questions in Region chat, Find people in recruit chat if need help with some mission... and ya warframe.market is great website if you want to trade ... You Don't have to spend money on Platinum... you can trade prime parts or mods and others tradable stuff that other players want and sell them for Platinum. It is in no way pay to win. Aside from Prime accessories and Tennogen you don't need real money for stuff.
Good Luck with the GRIND, Tenno!

Note: You can buy platinum or prime access if you want to directly support the developers. Prime Access is their way of selling latest prime warframe and prime weapons along with Prime accessories (cosmetics). You can farm for parts of prime warframe and weapons from relics instead of buying. Prime accessories are a limited time exclusive accessory set, you can also buy the accessories only.

Edit: Ya the 'content' Updates are quite slow i agree and it feels like it has nothing to do other then extra grind or riven rolls But only for the ones who have played all the previous content. For a new player it has a lot of content. You can always go ahead and play another game and come back to warframe. :)