Make sure you close/exit Minecraft before you start
1. Open your .minecraft folder (the place where the resource pack and mods folders are stored)
2. Find and open options.txt in your preferred text editor (notepad works)
3. Find and change the gamma to 100.0
4. Save and exit the text editor (important to remember to save)
5. If done correctly you will now have fullbright when you open the game again but if its still too complicated for you and you use windows there is a link to a video tutorial I found on youtube below (credit goes to the uploader of that vid). If you do not use windows simply search for a tutorial on how to install a resource pack (it should show you how to get to the resource pack folder and from there you should only need to go up 1 folder).
video tutorial (windows)
Note: I searched the forum before making this tutorial to make sure there was not a thread like this already posted.