You need to pay $99 a year to gain developer access. And be aware that you will have to deal with lots of bugs and other issues. Many apps won't work or will crash frequently. That's all just part of being a developer and using beta software.
You can also wait until the end of this month and sign up for the public beta program. They get access later than developers but it's free. Keep in mind you will still have to deal with frequent crashes, bugs, battery drain, non-working apps and other issues that are just part of a beta.
https://beta.apple.com/sp/betaprogram/
And remember that "bugs and other issues" sometimes includes bricking your device. I don't know if Apple has changed their policies since I was in the program, but I bricked a couple of devices and they told me I was SOL.