FB plus newbie

Hello! I have been following Kelli and Daniel since the start of this year on my sister's suggestion, but these past two months I've been really slacking on making time for workouts. I figured joining one of these programs would help with motivation so I'm giving it a try!

If anyone has any suggestion on what program I should try first - I'm looking to gain muscle and definition, not lose weight. I'm in my early 20s, 5'3'' and 120lbs. I sit A LOT due to school and working from home, if I don't purposefully get up and move I'd be sitting all day (it's horrible!) I've started to feel my lower back and butt starting to ache, I really don't want to develop back pain or a hunched posture.

I just completed a short video, the light toning posture workout for stiff shoulders, back and core and I already feel the soreness in my body easing up. I'm excited to get back into healthy exercising!