Don't necessarily need an app.
I'm not a gym goer but have a little bit of basic knowledge regarding muscular strength and weight loss.
You need to burn calories (more than you consume- so you may need to look at what you are eating in terms of content that you find on boxes/packets etc) to lose the "chubb", so treadmill work, steady pace, long-ish stints...
Know your limits - aim for what you think you can do to start with, then each time after, push yourself a little more in terms of time.
Eg, 10 mins, next session 11 mins...
To build muscle, you need to think where you want to do it.
Then, use machines/weights to do this.
Same principle as above - what you CAN do first...find your limit - then you have to increase weights. To build muscle, you need to do lift heavy weights but with fewer reps.
Put yourself a little routine together.
Steady run;
Upper body area
Lower body area;
Upper body area;
Lower body area;
Repeat - increase weights.
That's the basic principle...
Sure some regular gym goers may add a bit here...