First of all, download the Minecraft client software for your computer here (run the program and install it using all the defaults). Run the Minecraft launcher, login with your email address and password and click PLAY to go into Minecraft the first time (this sets up the game folders etc.)

Now download and install Java here (run the program and install it using all the defaults):

Now install the Forge client here (run the program and install it using all the defaults):

Next you will need to download the following mods:

Main modpack (extract and drop into the "mods" folder)
Millenaire mod (extract and drop into the "mods" folder)
Optifine Low (lower quality, but faster) / Optifine HD (higher quality, but slower) (drop into the "mods" folder)
Fastcraft mod (drop into the "mods" folder)
Optional shader (drop into the "shaderpacks" folder)

NB: The "mods" folder is %appdata%/.minecraft/mods

NB: The "shaderpacks" folder is %appdata%/.minecraft/shaderpacks