Welcome back, Tenno! 9 years is a long time so lots of contents to go through. There's an official quest guide you can check out: https://www.warframe.com/game/questguide It was a guide to prepare people for The New War so it's a couple years old but still good.
Like others said, go through the Star Chart and the quests. You now have two new player experience. The original way or The Duviri Paradox. Pros and cons to both paths, but if you didn't go through all the quests, it might be a bit disorienting. Personally, I would do the quests all the way up to The New War, but that would take a while. Once you cleared the whole Star Chart, you have access to Arbitration and Steel Path. I recommend doing Arbitration to make your frames and weapons stronger so you can tackle Steel Path.
Other than that, farm weapons and frames and do your MR tests. Regular Duviri Circuit is a good alternative to get a bunch of the frames that you might have missed. Other frames will still require doing quests and specific missions. Join a clan to access more weapons and frames without needing to farm for blueprints.
Keep an eye on this thread for free stuff. Right now, only things free are glyphs (you can also get my free glyph there, lol), but sometimes there are codes for goodies besides glyphs.
If you want a free Rhino Dex noggle, play the mini game here: https://www.warframe.com/anniversary#game-section
Feel free to add me as a friend in game. I'm always happy to play with players old and new. :D