For breakfast?

I do use protein shakes to, and yes your right It's better to get in the food balance. Protein shakes are recommended for the short term ie a few weeks as a kick start and Not the long term. I did use them a lot in the first couple of months as breakfast and lunch and a balanced meal in the evening and this did work. Now I use the but only now and again.

The reason they are recommended for short term is when people rely too much on them as a diet aid and not as a supplement. In some diets, people try to use these like a diet shake. I'll drink this instead of eating a meal. The result is that you're not getting solid food and you can end up gaining because you are on a liquid diet and your cravings increase.

I don't replace a meal with them. I use them to fuel and for combating muscle fatigue. Talk to any personal trainer or nutritionist and they'll tell you if you are actively exercising and your workouts are intense then shakes are a good idea. Even for the long term. I'm not a meathead and avoided them for the long time but I started using them after the advice of two personal trainers and a nutritionist.

Back the OP, it sounds like you aren't on that level of exercise yet, so my advice would be to just keep looking at what you are eating, concentrate on the cardio and also look at strength exercises you can do without flaming up the shoulder. I would also talk to a doctor. You may be surprised. Weightlifting may actually help it. The idea for lifting is that it strengthens the muscles and relieves pressure from the joints. So as you build some muscle, the inflammation may go away.

But once again, consult a doctor before pursuing that route.

Kudos to you for getting into your diet & exercise so early!

Unfortunately, you can't really spot reduce so if your body is hanging on to belly fat it may be the last place you're going to lose. I'd keep up with the cardio..cardio is a great calorie burner.. you could try some HIIT (look it up - many people use different methods) instead of steady state cardio to really rev up your calorie burn during your work outs.

As far as food & protein...

Because you're 18 which is when most males are still growing, I'd really suggest you get most of your nutrients from balanced meals. You can try adding a protein shake after your workouts but I wouldn't really suggest it as a meal replacement approach, and I would probably get the opinion of a dietician. You're still in a pretty sensitive time in your growth stage. I use Optimum Nutrition Gold Whey (they sell them at GNC) if you decide you want to try it out. It doesn't sound like you're doing any strenous exercise on your muscles (like lifting) so I don't know if a protein shake is necessary right now unless you really feel yourself getting sore after workouts.

Rocklion's right though.. some lifting may help with your inflammation. (i'd talk to a doctor or physical therapist about a possible approach). Strength training helps build muscle which also gives you that strong,lean physique that everyone wants and increases your metabolism to help you get rid of that stubborn belly fat. A good combination of cardio, strength training and calisthenics with weights would help. If you can't do weights because of your arm.. you can start with exercises that target your core so that when you do lose your belly, you'll have a tight core underneath (planks, crunches, bicycles, V-ups, Romanian twists, etc..).

If you're working on abs, I strongly suggest staying away from simple carbs and processed whites (table sugar, white bread, white rice, potatoes, etc.). Some people say staying away from dairy also helps them.. I have skim milk with my shake and cheese and yogurt as snacks so I think everyone's different. Processed snacks are also not the best idea.. cakes, cookies, chips, etc. You should try more nutritious snacks like fruit (not too much because it is a simple carb but has a lot of nutrients), almonds, greek yogurt, etc.

I'm usually pretty simple with breakfast but some of my choices are usually:

scrambled eggs with cheese, tomatoes and spinach
low cal whole wheat wrap with beans, hummus, fresh veggies, etc
steel cut oats
greek yogurt with honey, fresh fruit or granola
whole wheat english muffin
Kashi Go Lean cereal with skim milk
protein bar

The times that my stomach gets tightest is when I don't eat late.. I'll go through phases where I'll stop eating by about 7 pm latest. I eat dinner early and make it my last meal. I'll usually make sure it has a lot of protein and very little carbs. And then just have a shake or glass of milk after my workout since I workout late. Everyone's different though but you can try it.

Good luck!