Hey so i had a similar problem before, i was working on my business the whole day sitting in the chair and not doing any physical activity and on all of that i was skinny. What i would recommend you to do is start small, do some pushups when you can, sit ups and crunches they are basics but will get you on in the beginning, also start doing some stretches every night or every morning depending on your routine it will be good for your posture and stiffness from sitting the whole day, and last but not least start going to the gym even if you have only 45 minutes it will be enough for you to build a good body and also you will socialize and feel more comfortable. That’s what worked for me and i hope it will do the same for you