If you only have a studio mic (i.e. blue yeti) this will not work, you need a headset mic
Enter Fortnite and DISABLE voice chat
Exit Fortnite
Open your sound settings by clicking the sound icon in the bottom right of your desktop
In the playback devices tab and disable everything that you aren’t using to listen, and make your headphones the default device and default communication device
Do the same thing in the recording devices tab except this time you’re selecting your headset mic and not your stand mic if you have one
Once you have all of that done, press OK and reopen Fortnite
Re-enable voice chat and test it in a party