Reposting this class I taught last year that gives a crash course in plant identification. If you want an even better version of this class (consisting of four separate classes) then Backcountry Press still sells them (they're not mine to give away, unfortunately).
If you're just getting interested in botany and want to know how botanists can see a plant they've never seen before and know what family it's in and what other plants it's closely related to, this video explains it