To join a tournament in Fortnite, you first have to meet a few specific conditions. First of all, you have to enable 2FA (two-factor authentication) on your Epic Account. 2FA basically means that as well as entering a password to access your account, you’ll also have a one-time code sent by means of your choice: SMS, email, or a third-party account verification app. It’s fairly quick and easy to set up 2FA for Fortnite; follow this link. You may need to set up an Epic Account first.
The second condition is that your Epic Account level must be at least 50, which is fairly high but not super high. The idea here is to weed out spam accounts and ensure that all players competing in tournaments are experienced Fortnite players. Find your Epic Account level on the in-game Career tab, found on the main lobby menu.
You can check which tournaments are scheduled, and if any are underway, by going to the Compete tab in the Fortnite main lobby menu. You can check details on various tournaments here, but this isn’t where you enter an active tournament. To enter a tournament, you have to go to the Play tab, then the change game mode screen, where you’ll find active tournaments under the Discover tab in the Fortnite Competitive section. Select the tournament you want to enter, then go back to the Play tab and start a match as normal.