First you need to have a program like WINRAR, WINZIP or ZIP7, those are mostly free or they have 30 to check them out, then you double click in your file, and that's it. It is going to lead you to what's inside that file. A lot of people put several versions of the same book inside the file, whereas pdf, epub, mobi, lit html.
There are two ways to extract those files, you can click and drag the file to the folder you want or you may want to use extract. Now you can also extract everything from the rar file without opening, when you are in your explorer or whatever you use, touch your file with the arrow of your mouse and instead of clicking with your original button (usually the left) you are are going to click the right side button. After that its going to open a menu, you will have several options, within those options you will have the extract file.
Now, if inside the file you don't have a pdf file, you can use a program like CALIBRE, this program is completely free and easy to use, you can convert most of the extensions out there to the one you want.
There are also a lot of free programs to use and read the file in its original form. For instance, you happen to find a file with the extensions .prc, .mobi, or .epub, you can use a FREE program like FBreader for windows, its completely free, easy to use and light on your computer, it also comes with an ANDROID version of it.
If you find a .lit file you can open this with Microsoft Reader which is also free.
Hopefully you will find this info helpful, sorry for any grammar, mistakes long winded explanations, etc., but I'm falling asleep in front of the computer and cannot really 'compute'.