Navigate to
My Documents/Euro Truck Simulator 2 or My Documents/American Truck Simulator and find the game.log.txt.
Open the file, press CTRL+A to select all, press CTRL+C to copy all.
Go to https://pastebin.com/ , in the ''new paste'' box right click and choose ''paste'' to copy your log content there.
Press ''create new paste'' and copy the link of the page, post it on forum.