Just go slow and follow video tutorials that offer little alterations for less flaxable people/beginners. I use yoga with Adrienne, she's the best 🙂
Yoga is a combination of simple stretchings and core exercises. You could get started by simple poses like vrikshasana (tree pose), tadasana (palm tree pose) and Surya namskar (sun salutation). Then you could move on to some breathing exercises or pranayama like anulom vilom and kapalbhati. If you stay constant on your yoga routine then you will be able to see differences in yourself and an improved self concentration. Hope this helps.
Do not think too much. At least for me, yoga is that time of the day when I'm not concerned about anything else. Besides getting a bit challenging, take this task as something that can and will clear your mind. Be kind and give yourself the time needed to get better, little by little. Pay attention what the teacher is saying, using a mirror to help you if possible and let your brain completely relax, get empty and fill it with the satisfaction of doing your best in something that will be part of your self improvement.
I started with joining a 30 days of yoga Journey on Youtube and still 3 years later I"m doing a sequence every morning. Every januari there is a challenge on "yoga with Adriene" but you can try them on any month.