How to get a modded world in minecraft?

Modding minecraft is done by using minecraft forge. First what you have to do is create a new profile in the minecraft launcher with the version you want to mod (I recommend 1.6.4, the newest is 1.7.9 but not many mods are comparable with it) then run the game and quit once you get to the start menu. Then go to files.minecraftforge.net and download the latest version of 1.6.4 forge. Run it and you should be good to go. Now go to the launcher, you should see in your list of profiles a new one called forge. Select it. Reopen minecraft and you should see on the bottom left a few mods installed and a new menu that is called mods. Now quit. Now you are free to download any mods you want and apply them to minecraft. To do this, drop the file into the mods folder. To get to this, open the minecraft launcher, go to the edit profile, make sure it's forge. Open the change game dir option, you should see a few folders, one called mods. Drag and drop the mods into it and your good to go.