Ok. I will assume you aren't using forge. (If you don't know what forge is, just ignore that message!)
Heres how to install mods: First you get your downloaded file. Make sure you have a program like winrar, or 7zip. (an unzipping program.) Don't worry, they are free. Then you open the zipped file with your program. right click on it, then press > (Open with) The open > The program you installed to unzip files (winrar, 7zip, ect.) Then you just grab the modded file in their, with the .json file and the .jar file in it. then you type in %appdata% in your start bar (bottom left.) and go into the %appdata% folder. then you click on the .minecraft folder, then you click on the versions folder in the .minecraft folder. After you are in the versions folder you drag the mod file (with the .jar file and the .json file inside of it) into the versions folder. Than you are set. All you have to do is go onto the minecraft launcher, than you click on nw profile, then you will click the: use version button and scroll until you see the name of the mod you put in your versions folder. Than you click on that. Afterwards you can name the profile whatever you want. than click "save profile" at the bottom. There, you are done. to load up the mod you just click on the profile on your profile list and load minecraft. Hope I helped!

for most mods, download the version of forge that is for the version of minecraft the mods you want are for, then it will make a mod folder in your .minecraft, then just download the mods and put them in the mod folder.

What kind? Forge or ones like Optifine?