If you don't have any programming experience i would go with a 2D shooter aswell, especially because you wont have to mess with the third axis (for the most part)
I'm relatively new to Unity itself but I've a major in Computer Science and've been programming for a good 6 years now, so I'm able to pick stuff up more rapidly.
That being said, even with previous knowledge I still made my first games in 2D just to grasp the basic concepts of the engine. Brackey's videos helped me a lot with this.
A thing that I do in preparation for anything is to simply immerse myself in what i'm going to be doing. So if i'm going to make a 2d top-down shooter, I start playing tons of 2d top down shooters (can be flash games, free games on steam, w/e) so that I can get my mindset in the right place for the task ahead.
It really does help because you will have all these references and knowledge fresh in your mind.
If you need inspiration, watch Brackey's 10 years of game development video, Blackthornprod's 30+ games video and Sebastian Lague's 10 years of programming video:
https://www.youtube.com/watch?v=egukLtEhyP0
https://www.youtube.com/watch?v=T18X1HuawRc
https://www.youtube.com/watch?v=dae5KKQSqC0