I have some questions about modding Minecraft.?

For the majority of mods they will work in an existing world. For SDKs gun mod it usually works. But there is always the chance you mess up your save files or install the mod wrong and get a black screen. So back up your .minecraft folder before modding it. That way you just restore it if anything goes wrong. Worst case scenario you can just erase your .minecraft and re download it to play again. But then you wont have any of your saves.