Simple answer, I don't really recommend anyone under 18 using an ECA stack. You have a lot of options besides using stimulants especially since your young and should have a strong body and metabolism. Really, ECA's and stimulants should be used as that little "something extra" to help get you past your sticking points in my opinion.
also what are some side effects from taking a ECA stack that i wouldn't like (lose of sleep, strength going down etc.)
Stimulants like Ephedrine effect everyone differently, but most common side effects would be an increased heart rate, sweating, appetite loss, sleep loss, headaches, dizziness, etc. Some of these sound like benefits (appetite loss) but you would be surprised how many people forget to eat on these and lose a lot of mass.
Your main focus should be the exercise and proper diet. Doing the cardio sessions will definitely help, but keep in mind that 20 minutes three times a week is only an hour a week. If your stalling I would recommend boosting the cardio to around 30-45 minutes a session and see how your progress changes. Of course you can also add in more cardio sessions if your up to it.