Is it possible to take part even if you're a beginner?
It depends on the rules of this particular hackathon.
But I can say that participating in hackathons in my country doesn't mean you have to be proficient in Ethereum development or blockchain in general. Usually there are many categories of people who team up and work on a project doing different roles. So, don't be afraid, you can always join some team and be useful in one way or another. But if this is solo-hackathon, it might be good to learn something before participating. To start learning how to develop dapps you can start with cryptozombies, truffle pet shop, ethernaut, etc.