Ok, we'll take this in two stages, first, installing mods:
Mount&Blade will probably be installed to your C:\ drive by default, so follow this path in Windows Explorer (Not Internet Explorer): C:\Program Files\Mount&Blade\Modules
This is the modules folder. In there will be the Native module, the default game. To install a mod, unzip it and put the folder (for example, Hundred Years War) into the modules folder, just copy it over.
Now, judging by your lack of computer knowledge, making mods for M&B will be very tricky. Modders change the game using a set of scripts, known as the Module System. These are written using the Python programming language, and it helps to have a rudimentary knowledge of programming to use the system.
On the other hand, you could just create models (the items in the game) and textures (the graphics that make them look good) and get someone else to code them in.
One last thing. There are a lot of historical mods out there already, so unless your idea is somewhat original, it would be a bit pointless.
Hope this helps.