I firmly believe that being physically fit should be the foundation that every person's life should be built off of. Not only does it have an abundance of benefits to your physical health, but it also has a significant impact psychologically. Exercising regularly is proven as one of the best methods for both stress management and emotional regulation, not to mention recent studies that have shown exercise to boost learning capacity and brain activity in general (see Spark below).
To me, there are three main categories of fitness: musculoskeletal strength via lifting weights, cardiovascular fitness via running/biking/swimming/etc, and flexibility via static stretching/yoga/active mobility. A successful fitness regiment will encompass aspects of all three. I also believe that no matter how little you may know about a given category of exercise, with effort you can learn enough to design a plan and get started all by yourself. But rather than pretend to be an expert on any of these topics, I'll instead recommend some resources that have been the most influential for me.
80/20 Running: Run Stronger and Race Faster By Training Slower by Matt Fitzgerald
80/20 Running has been by far the most helpful resource in going from having zero knowledge on running and how to design a training plan, to having a fully outlined plan and understanding the importance of each piece within it. Not to mention that it's structured with running primarily at slow speeds, which, as the author covers, is actually optimal for performance improvement. After covering an abbreviated history on running and why primarily slow training is the way to go, Matt Fitzgerald covers in detail the different aspects of the training. What is perhaps the most helpful feature of this book is the training plan section, which contains complete programs to prepare for your first/next race, ranging from the 5K to the marathon. I'd highly recommend this book to anyone looking to get started in running, or for anyone looking to improve.
Spark: The Revolutionary New Science of Exercise and the Brain by John J. Ratey MD
As briefly mentioned above, Spark covers the benefits of exercise beyond the physical. It covers many different encouraging studies showing how and to what extent regular exercise can impact the brain, and the related benefits. This book provides great motivation to exercise for reasons other than to just stay in shape.
Bigger Leaner Stronger: The Simple Science of Building the Ultimate Male Body by Michael Matthews
Bigger Leaner Stronger (BLS) is the most practical book on lifting I've read. The fitness industry is so saturated with different schools of thought that it's difficult to determine what's actually legit. Mike Matthews also thought so, which led him to conduct a research campaign into fitness studies to find what actually gets results. This book is the result of that research. The main premise of BLS is that a lifting regiment shouldn't be overly complicated, and also shouldn't be a time drain. To some people the style might seem to be overly simple, but I found the simplicity to be its best feature. The book also spends a good few chapters covering diet, which many other books in the field overlook. Overall, if you only read one book on lifting, I'd recommend this to be the one.
Starting Strength: Basic Barbell Training by Mark Rippetoe
If you have time to read a second book on lifting, look no further. Starting Strength does an excellent job in breaking down the biomechanics of the different exercises you'll do in any lifting regiment. I use this book as the definitive guide on lifting form. While BLS does contain some brief excerpts on form, Starting Strength covers in detail how to improve form for many different lifts, and also why proper form results in less injuries plus better strength gains in the long run.
Becoming a Supple Leopard: The Ultimate Guide to Resolving Pain, Preventing Injury, and Optimizing Athletic Performance by Kelly Starrett and Glen Cordoza
The benefit of improving flexibility is twofold: it will boost the gains you see in other training, and it will make you feel better in general. Becoming a Supple Leopard is an encyclopedia-sized collection of stretches and mobility exercises that you can use to improve your muscle mobility, with stretches/movements hitting regions ranging from feet to head.
Yoga with Charlita (YouTube videos)
Yoga is also a fantastic way to improve flexibility. While I think the classes are much better in person, given the current circumstances, online videos are also a great way to practice. The linked playlist was created by my favorite yoga teacher, who teaches at the local fitness center I go to, and who coincidentally is also a professor of Spanish at Elizabethtown College, Dr. Charla Lorenzen.