Hello,
To start off, I have a few questions for you:
1. Does this occur in singleplayer?
2. What sort of crash are you experiencing? e.g. 'ets2.exe has stopped working' or 'fatal error' crashes?
3. Do you have any wheels connected?
If you answer the questions above, I can provide a more specific answer.
However, for now - please try follow my steps
> Go to Steam
> Steam Library
> Right click Euro Truck Simulator 2 / American Truck Simulator
> Properties
> Local Files
> Verify integrity of game files
> Wait for the process to complete, then attempt to launch MP.