Read Apple's Beta Rules & Conditions
If this is your first time trying an iOS beta, make sure you read through Apple's rules before you download the software.
If you want to download the beta right now you need a developer account. If you decide to go with the iOS 13 developer beta, make sure you take a look at Apple's beta website for developers.
This page goes over everything you need to know before, and after, you join Apple's developer program.
If you don't want to pay to use the beta, you'll have to install the public iOS 13 beta.
In order to download the public iOS 13 beta, you need to join the Beta Software Program. And like the developer beta, the public beta program has some rules you'll want to follow.
Before you download the iOS 13 beta on your iPhone, make sure you read Apple's FAQ on the Beta Software Program.