Free trials for a game usually take place over the weekend, but you're gonna need more time than that to explore all of Tamriel in Elder Scrolls Online - lucky you, Zenimax Online is hosting a free trial week from April 11 to April 18.
All you need to do is head to the Xbox Store, PlayStation Store, or Steam page for ESO: Tamriel Unlimited after 10am Eastern / 3pm BST and download the game. After that you'll be able to create an account and login free of charge. Should you choose to take part in the free trial week, you'll be given 500 Crowns (the in-game currency used for cosmetics, boosts, and loot boxes), and any progress you make will be carried over to the full version.
If you decide your adventures in Tamriel are worth continuing, you'll receive a discount on the base game or the Gold Edition, which contains the Imperial City, Orsinium, Thieves Guild, and Dark Brotherhood DLC packs. ESO no longer requires a subscription, so no need to plan out a monthly budget beyond the game itself.