Hello there! I don't live in CA anymore, but I used to and as a kid I used to fly fish the small rivers in the South Lake Tahoe area and have been fishing mountain rivers for a long time there and mainly in Colorado. From my experience the small creeks around there (Glen Alpine and Taylor Creek to name a couple) will produce a lot of fishing fun and are a great place to start for a beginner.
I've never been to the Truckee, but S Lake Tahoe area is in a somewhat similar area and about the same distance from Sac. So my two cents:
Large rivers can be a bit tough especially for a beginner. Smaller rivers have certain advantages: much easier to read water, easier to spot fish, easier to wade & cover water, easier to practice and understand the importance of presentation, and you will see the fruit of your learned knowledge quickly and thus can graduate to larger rivers with a solid foundation of fly fishing basics.
When fishing small mountain rivers/streams you have a lot of places for fish to hold. The runs and seams of the fast water and slower water on the side is usually a good place to cast. If a pool opens up and you have a nice rock in the river it is almost certain to have fish holding behind it and often in front of it. You can get some really good dead drifts with your dry or wet flies and if your presentation is good you will pull out many fish on a trip. They aren't always huge 8lb trout, but 10-16 inch fish should be quite common and a lot of fun.
To be honest even out in Colorado I tend to have the most fun on the smaller rivers just because of the ease of reading the water and finding holding spots for fish. I pull in way more fish on those smaller streams than when I go to the bigger S Platte or Arkansas rivers. That being said it is also a hoot to have a 20+ inch trout on the end of your line from one of the bigger rivers, but without the fundamentals I learned in the smaller mountain rivers around S Lake Tahoe and then in Colorado I would have very poor results from the bigger rivers. Even with my background and experience I catch WAY less fish in the large rivers than I do in the small ones (some of it has to do with the huge amount of pressure the big rivers in CO get, some of it is that it is harder to read the water and get your flies to the right depth and presentation).
So as a beginner I would recommend finding some smaller mountain streams with good rocks, pools, runs, and varying water characteristics. When you start catching 10-20 fish on an outting in those then you have probably learned enough to start venturing towards those bigger watersheds(rivers).
I have to say that if you are new to this go get this book: The Little Red Book of Fly Fishing. Very good book and straight to the meat of the matter. No wasted time reading about nonsense. It's basically just tip after tip and gives you a ton of knowledge to try out in a very short time.
So thats my advice, I hope it helps you and good luck!
