brunoWoWPs

No problem, all of us were Noobs sometime or the other!
 
Your root folder is located in: C:Program Files (x86)SteamsteamappscommonGrand Theft Auto IV
Your root folder is usually where the mods go to, unless they specify for the scripts folder, which is:
The scripts folder is a folder that created in your root folder for scripts that need ScriptHook.dll to work.
 
To run the game as Administrator, go to the root folder & right-click on "GTAIV.exe", select Properties, go to the "Compatibility" tab, & select "Run as a Administrator". Then, whenever you launch GTA:IV, you will get a popup from "User Account Control", just click "Yes".
You don't necessary need ONLY 1.0.7.0 version of GTA:IV to make anything work, there are mods that support earlier versions. However, since your IV is from Steam, you can rest assured that you have the latest version of IV (1.0.7.0).
 
 
First of all, to make any mod work, you need either ASI Loader (which replaces dsound.dll) or xLiveless (which adds 2 xliveless files). I would highly recommend the ASI Loader, since xLiveless remove GFWL, thus not allowing you to play MultiPlayer.
Get the ASI Loader from here: http://gtaforums.com/topic/394806-yaasil-yet-another-asi-loader/
 
Next I recommend you get the .Net ScriptHook. Most of the mods require ScriptHook (including LCPD:FR).
Get the .Net ScriptHook here: http://gtaforums.com/topic/392325-beta-gtaiv-net-scripthook/
Also, be sure to scroll down on that page & download Microsoft's .Net Framework 4 & Visual C++ 2010, you'll need these to make .Net ScriptHook & ASI Loader work.
 
Next you will need a Trainer, these help you with various stuff. I highly recommend the Simple Native Trainer v6.5. It has loads of features (including vehicles & weapons spawning, god mode for character & vehicles, Gravity Gun, Player Selector [allowing you to change Niko into any character in the game], spawning "bodyguards", & many many more) & is simple enough for the amount of features it has. I would recommend you read the ReadMe to get familiar with it.
 
That should do it.
Make sure you install the mods separately, that is, install one mod, start GTA:IV, go in-game (start the game & enter the game world), then quit GTA:IV, install next mod.
I know this is tedious, but it's very important, in case any mod causes a problem or starts overloading GTA:IV, you will know exactly which mod it is.
 
Also, (I'm not sure if this works in Steam versions, since I have the Disk version) I would suggest you make different "versions" of your GTA:IV folder, just go to one folder above your GTA:IV folder (C:Program Files (x86)Steamsteamappscommon) & copy & paste the IV folder (in this same place), you should now have a new IV "version" named "Grand Theft Auto IV(Copy)", rename it to whatever you want.  For example: I have 7 "versions" of GTA:IV:-
1) Normal version with basic mods for MultiPlayer,
2) One version with LCPD:FR mod installed,
3) One version with Graphics mod,
4) One version with LCPD:FR & Graphics mod,
5) One version with a Map mod,
6) One version with almost all Cars modded, &
7) One version which is the original CD version (1.0.1.0), so I don't have to be bothered uninstalling & reinstalling from the CD, if I ever need a "clean" version of GTA:IV.
 
 
EDIT: Wow! This is one loooong reply