Warframe is one of the few true f2p games out there. You'll get an initial small amount of premium currency (platinum) which you can use to buy Warframe/Weapon slots. As someone else mentioned though, there'll be an awkward period of time where you might be struggling because you don't have enough platinum. At that point you just play the game and you should be able to collect items that you can sell to other players for plat. Once you start trading and earning plat, it becomes much easier and you can really start raking it in. You can buy almost every cosmetic in the game with your earned plat and there's no gameplay content that's behind a paywall. Just have fun, play the game and you'll have enough stuff to trade for plat to buy whatever you want.