'Thesis' is what you write when you are studing for a Phd. I suspect this isn't what you mean. Having said that, I have no idea on how complex the game needs to be or how long you've got. Sounds like you've left it to the last minute!
The Game Proposal
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Working out gameplay that is challenging, yet fun is difficult. If you don't have to be original, I would suggest reworking the theme and gameplay of an existing game. If it has to be original, that's really hard, as you won't know whether it plays well until after you make it.
Consider the various game genres. e.g. Platformer, Shooter, Role Playing, Strategy, Racing
Decide on a Theme: e.g. Space, Western, Fantasy
Decide on the Implementation: 2D versus 3D. If 3D is it first or third person view
Consider Level Design. There's a book 'Game Level Design' by Ed Byrne, but if you've only got a week, I doubt you've got time. What is the level goal, how is it scored; What power ups; How do you get damaged. How does difficulty progress with Level.
Think of a background story to set the game in context
If you get the proposal in successfully, it's time to move on to programming. Hopefully you have more time here, as you will need it.
You will need to learn to program in Java first, then when you've got the fundementals, move on to game programming. I suggest you get some books, download the java sdk and work through some of the examples.
Java Programming
http://www.amazon.com/Head-First-Java-Brain-Learners/dp/0596004656http://www.amazon.com/Introduction-Java-Programming-Comprehensive-Version-GOAL/dp/0132221586/ref=tag_dpp_lp_edpp_ttl_exJava Game Programming
http://www.amazon.com/Developing-Games-Java-David-Brackeen/dp/1592730051/ref=pd_sim_b_3http://www.amazon.com/Killer-Game-Programming-Andrew-Davison/dp/0596007302/ref=pd_sim_b_1
