First of all get yourself a USB to TTY cable. For example like this one here: Adafruit.
Dont try to buy it on ebay from China. It may cost 3 € less and even if they say it gets delivered in 5 business days, you will have to wait much longer. And have to pay customs. In my particular case, the Case had to pay nothing, but first the swiss customs checked it and after that even the german customs. Just buy it from your local vendor and support small business owners.
Connect it correctly with your Pi:
|red, free, black, white, green|
Just do it right. The little piece of Paper telling you what is what, is called the Raspberry Leaf, go and print one for yourself.
Now the important Part:
What operating System do you run? If you are using Windows 8, (no driver for you). I would suggest you to use a live cd of Ubuntu. Or another PC.
I myself am using Ubuntu Linux so, my commands would not completly apply to the Windows guys. Be advised and just use Putty.
So fire up your Terminal and here you go:
install screen on Ubuntu:
sudo apt-get install screen -y
Next open your terminal and type in:
Now search the Output for something like /dev/ttyUSB0 and use it in the next command:
screen -L /dev/ttyUSB0 115200
This is it.
You have established a serial connection to your Rasberry Pi, via a USB to TTY or USB to R232 adapter.
Now you can start using your pi, even if you dont have a ethernet-cable.