I think you should join it earlier but no problem. If you are interested then go for login here
https://next.splinterlands.com/login
using your browser.
Enter your hive account and Hive posting key. Purchase #spellbook( cost $10) with hive/
If you are not much aware then use practice battle to learn. There will be some Monsters and summoners to play but earn reward need own card and summoners. You can purchase it from here..

https://next.splinterlands.com/market/card-sales
Let me know if there is more queries.