Well, the best answer you can find in the Holy Quran.
Quran says in Surah 60 , Verses
8. Allah forbids you not, with regard to those who fight you not for (your) Faith nor drive you out of your homes, from dealing kindly andjustly with them: for Allah loveth those who are just.
9. Allah only forbids you, with regard to those who fight you for (your) Faith, and drive you out of your homes, and support (others) indriving you out, from turning to them (for friendship and protection). Itis such as turn to them (in these circumstances), that do wrong.
killling innocent being (irrespective of muslim or non muslim) is one of the biggest sin in Islam. killing one innocent is just like you killed whole of humanity.
Quran says in Surah 5 verse 32
"On that account: We ordained for the Children of Israel that if any one slew a person - unless it be for murder or for spreadingmischief in the land - it would be as if he slew the whole people: and ifany one saved a life, it would be as if he saved the life of the wholepeople. Then although there came to them Our messengers with clear signs,yet, even after that, many of them continued to commit excesses in theland.
Hopefully , these verses of the Quran clarifies you to whom we should fight and whom we cannot. We should pay attention to those people who just quotes thing out of context and some time wrong text and try to say that Islam allow to kill innocent non muslim. Islam does not allow to kill innocent peoples whether muslims or non-muslim , and teaches us to treat them well.
Also at the Time of Prophet Muhammad Peace be upon him, there are people who are fighting them and try to kill them but when Prophet over come those , what they did. Forgive them all. That what the behaviour of Our beloved Prophet was.
Donot just simply what media says or what some ignorence people do, even some people kill innocent childrent (even muslims) and try to associate it with Islamic war. So donot pay attention to such false claims.