Hi and welcome. Best way to start is to jump on the server, there are people mostly around 19:00-22:00GMT (i think, i'm so sporadic that i have no clear schedule but on those times i've seen the most people..). Remember to register at Prestogp.com so your laptimes are registered properly.
Skins:
Good place to get started is NoGrip, there's plenty of skins, google will guide you to a number of sites but community uses NoGrip the most when it comes to skins... But it doesn't matter where you download, the place to install them is: My Documents\SimBin\Race 07\Customskins. the folder structure doesn't really matter, just unpack them there, if you want you can make your own folders where to keep skins specific to one league or car class. Less you have unnecessary files in Race07, the better. That goes for mods too, keep only the ones you use often.
Mod folder structure:
Vehicle mods will go to Steam\steamapps\common\Race07\Gamedata\Teams. Very common way to organize mods is after you install them, make a folder on your desktop (something like 'MOD storage' and move the mods from \Teams folder to that storage) You need to this when the game is closed, otherwise it might crash as it loads all the mods when it starts... Same goes for tracks and skins too.
Addon tracks goes to GameData\Locations
Check before you unpack track and cars how the modder has packed them. Most vehicle mods are packed so that you can unpack them to Race07 root folder (...steamapps\common\Race07 ). A lot of track addons are packed so that they are installed straight to Race07\Gamedata\Locations.
Setups are in My Docs\SimBin\Race 07\UserData\Carsetups\ and then you have bunch of track folder, setups go in to these. Note, after you have installed a new addon track, start the game and load the track, this will create the track car setup folder.
Creating skins is better to search the internet or here in RaceDepartment, there are plenty of tutorials.
Same goes for making mods... I'm a modder and trackbuilder myself and it surely is not something one can explain in one post, unless you want to read 30 pages, now.. (it's taken me 4 years and i'm still a newbie..)
But i can give you help and maybe point you to the right direction.
PS: Extra tip, install XD from vitumo.de ... We all use that nifty little telemetry tool, follow the instructions.