How to get Java JAR file to launch?

Every time I try to download a mod for minecraft forge, I prompts me to "Check the console for possible error messages." Help please and thank you!

You… Don't… Launch mods… THE ONLY one that does launch, even without forge, is Optifine because it's not really a "mod," persay, but instead inhances your FPS. The rest, you drag that file to %appdata% into the mods folder. Look it up on youtube!