About 6 months ago, while playing basketball, I landed awkwardly and tore my A.C.L. My doctor told me that if my legs were stronger, my injury would have been less severe. Just two months ago, I started to train my legs again, squatting just the bar and eventually working my way up. I've still got a long way to go before I can play basketball or even run again.
It's OK to be a little sore. Your muscles might feel achy or tired the day after a tough training session thanks to DOMS, or delayed onset muscle soreness. When you strength train you're causing microscopic damage to the tissue that will be repaired, that's how you build stronger lean muscle. Speaking of repair and recovery, though, rest days are important. "If you constantly break down muscle without a recovery period, you won’t give the muscle fibers a chance to repair and build back stronger,” explains Davis.
JDB, great tip on cardio before weights. Having trained in a boxing gym for years, this is exactly how I would start each training session, even if I was going to focus on weights that day. Use to start off, 3-4 rounds on jump rope in the ring, alternating between fast high knees and then slowing it down. It was all continuous for about 10-12 minutes. Would then continue w basic calisthenics (mountain climbers, bw squats, burpees, etc.) for another 3-4 rounds. This was all done w boxing timer running in background. Would finish “warm-up” w some kind of pull-up/chin-up, dips, and hanging abdominal work. Then it was time for weights. This just overall improved my general conditioning/movement and never felt that my strength diminished when doing this. Actually, felt stronger due to increased blood flow.

Resistance training is any exercise that causes the muscles to contract against an external resistance with the expectation of increases in strength, tone, mass, and/or endurance. The external resistance can be dumbbells, rubber exercise tubing, your own body weight, bricks, bottles of water, or any other object that causes the muscles to contract.
One of the most persistent questions floating around the minds of many aspiring bodybuilders is “What is the best way to train to build muscle?”. The answers are as varied as the numerous pieces of gym equipment that occupy any fitness establishment. There are many choices as to the best exercises to use, how many sets and reps, how many days per week to train and what type of program to follow.
When lifting it is essential to focus your mind completely on the muscle group you are attempting to work on. This makes sure that you are actually using the target muscle to lift the weight rather than accessory muscles or even momentum. This skill will develop with time, and as you progress in making that mind to muscle connection, your workouts will become much more productive and efficient.