Hey dude,
Welcome! I recommending watching tutorials online and other videos to understand how this brilliant mod works. It is entirely up to you how you play but I do not think you can join a specific police station and will have to choose as you please. Do not quote me though! With regards to mods, it is a really simple process. It just involves dragging and dropping the files, into their required folders. Most mods will have a step by step guide that explains where each file goes.
Always ALWAYS back up your folders before you embark on adding any plugins, scripts, character and vehicle modifications. This will save you many headaches with things going wrong as this can easily happen. Not all scripts and plugins are stable with each other. Unfortunately, crashes will soon become the norm for you. If you have any issues, upload the ragehookplugin.log file anytime you need help in the appropriate section, on the forums.
Enjoy!