Taking lessons, especially something like an ASA 101 "learn to sail" or basic keelboat course, is a great way to get a solid foundation for your sailing career. It will give you the basic theory, tools, vocabulary and skills that are required of a sailor, and these will apply to most boats.
Going out with experienced sailors can be a good learning experience, but I'd caution that some sailors are horrible teachers, regardless of how good a sailor they are. Also, some very experienced sailors do not know the proper techniques and will teach you some very bad or dangerous habits if you're not careful.
Between an ASA 101 type course, a good book like Dave Seidman's The Complete Sailor, and your own boat, you should be able to learn quite a bit.
However, daysailing does not require the same skills that cruising sailors require. Route planning, navigation, and many other skills are often not required when just daysailing, and if you're interested in making longer voyages, then you'll need those skills as well. The USCG Aux, USPS, and other organizations often hold courses on things like navigation, which I'd highly recommend you look into. I'd also HIGHLY recommend you get Richard K. Hubbard's book, Boater's Bowditch, which is basically Bowditch re-written for the small craft navigator.
I'd point out that river sailing can have some challenges that are often not seen on lakes or even on protected bays...