Download the Litecoin client if you don't already have it: https://litecoin.org/
If you do have the client already, copy the binary to a new folder!**** Back up your existing wallet first, just in case! ***
Make a new file: litecoin.conf
Add a line in the file: testnet=1
Windows: make a new file: run.bat
Add a line in the file: litecoin-qt.exe -datadir=%~dp0
Doing the above will make the Litecoin client save the files to where you run litecoin-qt.exe from. If you don't use the -datadir flag, it might overwrite your existing wallet!
Linux/Mac: run the binary using the -datadir=/path/to/directory/of/binary
* If your binary is installed in the system, you can just call it with the above flag without copying the file locally.Run the client in testnet mode and get your testnet address.
Post your address here and I'll send you 500 coins! Get coins from http://litecointools.com
Here's an address to test your coins with: mvoZ3sQLrpeYGR754cyc44ny5gXENPLcrC - you can send them around to other people who post in this thread too!