Most Minecraft mods will have detailed installation instructions in their respective threads, however, there is one basic, traditional method to installing mods, that most basic mods will use.
1. Download the mod (obviously)
2. Open the "run" box by hitting Windows+R (you know, the windows key)
3. Type "appdata" or "%appdata%" if you're using XP
4. Navigate to "Roaming" and then ".minecraft"
5. Open up the "bin" folder
6. Locate "minecraft.jar" in the bin folder and open it as an archive with an application such as 7-zip or WinRAR
7. Drag the required class files as specified by the mod from the folder where you downloaded the mod to, and place them into the minecraft.jar archive
8. In the minecraft.jar archive, delete the folder titled "META_INF", otherwise Minecraft will crash when you launch it.
That's essentially it, however, some mods will require that you place folders or files into the "resources" folder found in the .minecraft directory, however if this is required it should state so in the thread or a readme file. It is integral that you read through the thread and all of the instructions that the modder has left for you, so you know exactly what to do. Many mobs have pre-requisite mods, such as ModLoader, AudioMod, or Forge. If they do, the thread SHOULD direct you towards those mods, and instructions for them. I know that AdventureCraft, for example, is a very complicated mod and likely has some detailed instructions and required pre-requisites. I've never used AdventureCraft myself, but I am very familiar with Minecraft and modding Minecraft. I would suggest reading over the instructions found the in the AdventureCraft thread in the modding section of the Minecraft forums, however if you have any additional questions, you may feel free to PM me.