A good way to get started is to download and install Eclipse on your computer. This will let you edit the source files and run the code. At this point you can go to the deck editor and create a very small deck with just enough cards to test the new card to see if it works the way you expected.
Once you have a working card you can post the code here in the developer forum. At some point you will be ready to take it to the next level which is to install
Subversion and Subclipse and this will allow you to upload and download changes from your local copy to the head version located on google's SVN.
As you become more familiar with the code you will notice that dennis likes to create the frame work needed for certain types of cards. Recently he added the code for
X cost spells and MultiKicker. He then got busy with other things and I took the opportunity to use his code as a template and modified it to the point where we had several other cards that could be added.
There also is code already in place which can be modified into a keyword which in turn can allow the easy addition of other cards. You can read through the cards.txt file and find numerous cards that are either implemented partially or completely through these keywords.