Expect to learn something ^^. Because that will happen :). Game jams are super cool to get a feeling about what you are good at and what you need to improve. Expect that TimeManagement is something you are NOT good at ^^. But knowing that, here's a tip: make yourself a schedule! You have 72 hours, 3 days. You need to sleep because you will get unproductive when too sleepy so plan about 7h sleep every day and plan another 6 hours for eating relaxing, taking a walk, whatever. So basically: from 72 hours, plan that you will use 36 hours for the jam, even if you will have more time and sleep less. This is a simple trick to set your goals achievable.
Next. Instead of planning 36h for your game now, plan 24 hours! Why? You should reserve 1/3 of the time for playtesting, bugfixing etc.
And the most important thing of all: plan a super simple game as your first game! Think about "guess my number" or "memory" or "klick as many bubbles in 1 minute as you can", "avoid falling objects by moving left or right", something like that. Focus on one game mechanic ( like klicking or jumping ). Why? Because think of this: if you make a super simple game with only one level you may have a playable game by end of day one! Then you have a lot of time to build on that! Add another level or better graphics or take the time to find some free music or a nice font to use it in the game. A jam should always focus on finishing the game, not making the next World of Warcraft in 3 days ^^.
As for the schedule: write down a very simple game idea (simple means: easy for you to make). Then write down everything that needs to be done, then bring it in a time order and write a number of hours on every task. Look at the list again and strike out everything that is not absolutely necessary. You can add those things back later if you have time left.
Now start and keep an eye on your time. Every task that takes longer will kill some task at the end of the list. So try to keep I time by again keeping things simple.
A schedule for a 2d game where you click on enemies that pop-up to score could look like this:
1. Create a simple enemy sprite (just a red square) (15m)
2. Setup your project in the language that you have chosen (2h)
3. Create a level with some enemies spread at predefined positions (2h)
4. Make enemies disappear when clicking on them. (1h)
5. Give the player points on the score whenever he clicks on an enemy (1h)
6. Add a scorecounter to the screen that shows the current points (1h)
7. Show a Game Over screen after 1 Minute that shows the final score (2h)
8. Show the one minute count down during the game on the screen. (2h)
9. Add a replay button to the end screen to restart the game on click. (2h)
10. Draw a more sophisticated enemy and replace the red squares with this image (1h)
11. Make a start screen with a start button that will start the level. The screen shows some nice image and has your name written somewhere. (2h)
12. Add a background to the level. Maybe an image of a forest or a pirateship or something. (1h)
13. Find a free to use sound, dowoad it and build it into the game and play it when an enemy is clicked.(2h)
14. Make enemies appear randomly over time. (2h)
15. Find and add some free music to the level (1h)
16. Find and add some free music to the start screen (1h)
These are about 24 hours! Of course the hours per task are very roughly estimated but it's better to overestimate here. Notice that you would have a playable game at position (7.) after about 10h! There you would have a game that gives you one minute to click as many red squares on the screen as you can in one minute and then gives you a score. To replay you would need to restart it and ut would not be pretty and without sounds but it would be a game. Everything that comes after that is up to you. After just 10 hours you would have something to submit if you need to.
So I hope this helps a bit. Of course this is not the only way to approach a jam but it is one that helped me on my first jams.
Have fun!