I think the best place to start is to grow some plants that you're interested in. I use OneNote to jot down research info on things that I'm either growing or foraging but you could also start a physical notebook. It's fun to learn about the native range and habitat that your house plants are used to.
I took this course for some more detailed biological info on plants and how they interact with the world https://www.coursera.org/learn/plantknows
I like to keep an eye on this site https://phys.org/biology-news/ because it usually has info from new plant studies that are coming out and sometimes just fun info.
If you're into podcasts and blogs http://www.indefenseofplants.com/ is my favorite. Their blog posts are good little nuggets of knowledge. A few of the early podcasts can get a little dry but you can learn so much good information here in a way that's a lot easier to remember.
look into getting a tree identification book, they're always good to have for reference and then go for a hike, you can even walk around your neighborhood and practice identifying trees. Then you can go to /r/whatsthisplant/ if you get stuck on one or are unsure if you've found the right ID.
There are also more specialized subreddits depending on what you're interested in diving into.
/r/gardening/ is for personal yard gardens and questions discussion related to them
/r/marijuanaenthusiasts/ is for trees
/r/bryology/ is about moss
/r/Lichenology/ is about lichens
/r/savagegarden/ is about carnivorous plants
/r/foraging/ is good for helping to ID edible plants and mushrooms you find in the woods or to see what other people are getting from the woods
/r/houseplants/ is for indoor plants
I hope this is enough to get you pointed in a good direction. Maybe your area has a garden club you could join because I found just talking about plants is a great way to learn.