How to determine other half of chemical equation?

So I'm very confused about balancing equations right now. And what I mean by the above question is something like Write the balanced equations for these expressions: H2 + O2 KClO3 KBr + MnO2 + H2SO4 AgCl + NH3 Cu + H2O What's on the other side?? By no means do you need to solve all of those (except maybe the third one, that'd be really appreciated actually), I'd actually appreciate if you could show me the process and I could do it on my own, but currently I'm just very confused as to how you get the other half. What I understand now is like AgCl + NH3, you just switch it around right? So it's now AgH3 + NCl and then you balance charges and balance the whole thing, or like Cu + H2O you just switch is so it's like CuO + H2 and again balance charges, then whole thing... But for like KClO3, I know that KClO3 ---> KCl + O2 and from there you balance (thank you wikipedia), but how do you know it's not K + ClO3 or KO3+ Cl? I know that might sound like a really stupid question, but I'm having a hard time figuring it out. Or H2 + O2 ---> H2O (not balanced), how do you know it doesn't form H2O2, is it just common sense, something you need to memorize or what? And the Kbr+ MnO2 + H2SO4 I'm just completely lost. Sorry for such a long description, I'm just feeling very panicked, it's kind of like being nervous, I talk a lot. But it's just because this seems so simple and I don't get why I'm not understanding it. Thank you for all help, it's much appreciated.