It is extremely difficult to lose fat and gain muscle at the same time. It is usually limited to small gains. The most successful way takes some patience; you need to lose the same 5 pounds over and over again taking about a month to lose it and a month to gain it back each time, lifting weights the entire time.

But you might be happy firming up and exposing what you have already. That's pretty much what I did.

IME, the best gentle intro to working abs is using an exercise ball. On your knees, sitting back on your heels, elbows at your side, hands on the ball about 2/3 of the way up. Roll it out it front of you, letting your hands roll off the top and forearms roll on. When you get extended and can feel it in your abs, hold there for a couple of seconds before rolling back. Over the course of several days, start holding the position longer and doing more reps.
After having done that a while, instead of sitting on your heels, start from leaning forward with your hands on the ball with it a little ways out if front of you and as you roll it, straighten your legs to go into a plank. Hold the plank longer and longer and do more reps over the course of several days.
Then moving on to other abs exercises won't knot up your gut and leave you in pain. For whatever reason, you can push abs too far without realizing it, impressing yourself because you are in some much better shape than you thought. Then the next day it hurts to do anything but lay on the bed curled up in a ball.