What you need; WinRar/7Zip, the mod file, and in some cases, Modloader. You obviously need Minecraft.

Basically, what you need to do for most mods is, download the mod file, drag it to your desktop, right click the mod file, then hover over "open with" then click WinRar.(7Zip also optional.) Then, go to your start menu, and search this, %appdata% , there should be a folder that is called roaming, open it, then open your .minecraft folder. Then, go into your bin folder. When you are in there, you should see a file called minecraft or minecraft.jar. What you need to do is, right click the minecraft or minecraft.jar, then hover over "open with" then select WinRar.(Again, 7zip is also optional.) You then should see a crap ton of files. THIS IS THE MOST IMPORTANT PART! DELETE THE META-INF FILE! Now, go to the mod that you downloaded, and click the top file, not the .. file, hold down you shift key, (up arrow) scroll down, and click the bottom file, all the files should be highlighted now. Now, drag all the highlighted files/folders into your minecraft or minecraft.jar folder. Close down all the folders you have open, then open your minecraft, log in and there you go! If it says modloader needed on the mod page, you just install modloader the exact same way as the mod. Put in Modloader first. Hope this helped!

Link to get WinRar: www.rarlab.com