You can try using this command in geth to get started with the testnet:
Just make sure that you are located into the same directory where geth is into your command line, before running the above command. Just use the cd command to go through directories from within your command line until you find the location of the geth executable.
Also, if you want to sync faster, id suggest you use the --fast command as shown below:
You will need a testnet account in order to receive testnet ETH. You can do so by running the following command:
geth --testnet account new
Hope this helps