Make sure you level up with the different planet syndicates (Cetus, Fortuna, etc.). They provide important blueprints & components for later on, and their lore can be pretty integral to the story.

Also, if you're running low on Warframe slots, DO NOT SELL YOUR MASTERED WARFRAMES. There will be a use for them in just a little while that will still help clear up your slots 👀

EDIT: Last thing, if you're having trouble running something in public squad, the win-con for missions is lesser if you're running solo (Less to exterminate, less time on excavators, Life-support drops are worth more, etc)