this is my first game jam and first time attempting to make a complete game
* Make it short first. If your idea is to have a whole set of levels, like a world in Mario 3, cut it down. Aim to make one level first. Most people won't play your jam game for more than 2 minutes
* Focus on the important tasks first. I remember back when I did my first game jam I spent a whole day making a shadow system, and by the end I had to rush the combat system which was the most important part
* Plan out your game. Write down your ideas and mechanics in a way that seem doable
* Make your game easy to finish. For example, put checkpoints and avoid giving game overs
However much you think you can get done in a weekend... cut that down by at least 50-90%. If you've never made a game before, you'll be surprised how much time things take to get finished --- AND how many little things you'll be working on that you didn't even think about.
So, as 7Soul said, make it short. Keep your idea simple and just try to get something playable. Games are a lot of work, but are very, very rewarding when you do get things together.
Hey - this is my first game jam too, so don't feel like you're alone. The only thing I am aiming at is just releasing the game, no matter how scuffed it is - and I urge you to do the same. There are very good suggestions above, but for my first jam - I'm keeping my game simple. I wanted to do a roguelike but... I don't think my skills are quite there, and that is gonna balloon out of porportion really fast I'd imagine! So yeah, don't be afraid to release your game.
I'll have only one thing to say: do not overextend your scope! Plan to make a small game, extend/add features if you have time for it :) It's a common mistake to plan a too wide scope and achieve nothing at the end.
Also, create, test, build, test. And keep your builds somewhere in case you break everything and have to revert/have to upload cuz it's time.
Happy jam!