Both are fairly simple,
If you have bought the game through steam Option one will work for you.
Step 1) Find the game in your game library and click on it,
Step 2) Click on the "Browse The Workshop"
Step 3) The interface that shows afterwards is where you find the mods, all you need to is type in the search bar too see if someone has made the mod you would like. Or if you don't have a clue in which mod to get you can click "most popular" then a drop down menu asks you "today" or "this week" this is where you find the good mods.
Step 4) Click on the mod you would like to use then click " +Subscribe " this will add the mod too your game upon start up.
Video On Step One: http://www.youtube.com/watch?v=htISWTaXi6E&feature=yout... (32 Seconds long)
This option is more longer to do (and can manage mods you have already installed to your Skyrim)
YOU WILL NEED AN ACCOUNT TO COMPLETE THIS OPTION
Step 1) Go To http://www.nexusmods.com/skyrim/?
Step 2) Register an account and install Nexus Mod Manager
Step 3) Run the program. It will ask you too login after a minute or so of loading.
Step 4) This will appear, click the tick on all games found (I have fallout 3 and Skyrim so they both were found) Simply click X on the other games if you don't have them and click continue
Step 5) Click this link (make sure you are still signed in) http://www.nexusmods.com/skyrim/mods/categories/?
Step 6) choose the type of mod you would like and browse. (for example i'm using "Textures Or HD Packs")
Step 7) Find a mod you like (I already have one I liked so I just searched for it)
Step 8) Follow Video To Finish
Youtube Video: (Step 8 and onward)
Thanks for reading this, peace out