A good warm is always an ideal to begin your sessions with. Jumping into intense training with no warm up is an idea that you may be able to get away with in your teenage years, but once you are older and your body becomes more unforgiving you will have to give proper warming up more respect, if you wish to be able to continue training your whole life (which is what you should want if you want optimal health).
Movement specific warm is a great idea if you start light. For eg if you are going to bench press, then some very light sets of 3-5 reps to get the target muscles warmed up is a good idea. However on most occasions and definitely on some occasions like when you are too stiff, too tired, its too cold, etc you may want to start off with joint mobility work to get those joints ready for the heavier action that is to follow.
Below is a whole body joint mobility warm up video featuring my training partner Samrat Sen, to get you started, and it requires no extra equipments:
If you train at home then you can simply do a follow along with the video to get you started before you work out. Most of the exercises are done for about 10 reps, and are shown from the front and the side for you to be able to get a proper understanding of them. You should spend as much time with each move as you feel you need to warm up that joint. So if you feel that the knee exercises require more than 10 reps, then go ahead and take as much time as you need. Again, each will have his or her individual requirements.
It can also be a great lighter intensity workout on your off days if you do it for a high number of repetitions. This way you can keep your mobility and reduce some of the stiffness that you may feel between workouts
