You need this: https://geti2p.net/en/download
If you're on Linux I highly recommend using i2pd instead, it's far better.
With either one of those just install it, run it, and give it a few minutes to connect to the network.
By default it'll share some of your bandwidth but if you'd prefer it not it can be disabled in both program's settings.

Then in order to access sites through it you need to use it's HTTP proxy: https://geti2p.net/en/about/browser-config
Both the version in the link I sent and i2pd use 4444 for their port.

If you need any more help feel free to DM me on Matrix at @butteredcats:catsarch.com (I'm also in the BreezeWiki server) cause this isn't emailing me with responses for some reason.