Sedentary lifestyle workouts?

Hello. I am F25, I’ve lived a mostly sedentary life as I work from home on a computer. I am looking for at home workouts to improve my core strength and posture, as well as loosen my pelvic floor; I am struggling with even the most basic of exercises. My core is ruined from slouching, and my back and shoulders are terribly tense from the same. I hope hoping to find some exercises that will be gentle enough for my nonexistent core to handle. I have a set of dumbells going up to 20 lbs, a yoga mat, and a lot of time on my hands. 😅