Good news! You can't become a heap of muscles! No-one "accidentally-Arnold" when working out. It takes YEARS AND YEARS of full dedication to get to a bodybuilder physique.
Surprisingly everyone's belly pokes out a bit. That's because you have organs in there. When you tense a well built six pack you push them back and flatten out your chest. This look can be achieved easily by maintaining good standing posture.
Firstly: Go see a doctor about your spine condition, show/explain the routine you want to follow and ask if it is suitable for someone with your back condition.
Looks like you need to do 2 things. Keep/Reduce your body-fat percentage and put on some muscle. Good news for you is that because you are a beginner you can do both at the same time.
With regards to reducing body-fat percentage I recommend 3x7 fork put-downs. Just eat less or just enough calories. Track calories using a food tracker and use IIFYM.com to calculate how many you need in a day.
With regards to putting on muscle:
- stronglifts.com/5x5 - Complete this program first. There is a helpful app and lots of videos to instruct you on form and educate you about weightlifting.
- Once you have been working out for around 6 months and you have stalled on most of your 5x5 lifts (following the de-load schedule). Then consider some of the following: 5/3/1, Texas Method, Greyskull LP, Jacked & Tan 2.0, PHUL, anything from https://www.reddit.com/r/Fitness/wiki/programs
In targeting abs specifically (in addition to the above) get yourself a Ab-wheel and do kneeling and standing variations. Build up to 5 sets of 25.