Welcome aboard! There are a lot of resources on this page.
To start, you may try clicking on the 5 tabs near the top of each page, especially "Daily Magic Spells" and "Monthly Contests". For Daily Magic Spells, you can try the daily problems. It is completely free.
For ZIML Monthly Contests, you can participate the contests during the first weekend of each month, choosing a level that is suitable for you. There are 5 divisions: Elementary (E), Middle School (M), High School (H), Junior Varsity, and Varsity. The January 2025 contest will occur this weekend. You can spend 1 hour's time during Friday, Saturday, or Sunday to take the contest.
Also you can try "Practice". It includes sample ZIML contest, and also includes past AMC 8/10/12 and AIME problems since 2000; you can take any of them for free, under real contest conditions (timing etc.).
I'm glad that you joined us, please keep exploring and enjoy!