Need help figuring out what to do

Hi, I'm still relatively new to Warframe. I unlocked the Steel Path and am working towards getting a few more Warframes by doing quests, but when I try to do the Steel Path, I end up getting obliterated, what should I do to improve my damage and health? I've heard of Riven mods, but I'm unsure how to get those or how they work. Any help would be greatly appreciated, thanks!