@Tsonga87 You have to apply to Merch using your regular Amazon account. They'll ask for your bank and tax details and a statement on why you want to join Merch, any experience you have etc. There are lots of videos on YouTube about applying for Merch but acceptance to the program is seemingly very random. For me, I had a long-standing Amazon account, told them I had a brand and a niche that I wanted to focus on and explained that I would take copywriter and trademark issues seriously. Some people just apply with no experience and get in. Others are experienced designers with brands and don't get accepted. The only way to know is to apply.
If you do get in you get 10 slots to put up designs. One slot equals one t-shirt, pop socket or phone case. If you want to put up a sweatshirt with the same design as your first t-shirt that takes up another slot. They don't sell as well so it's recommended to stick with t-shirts in Tier 10. You can only upload one design per day in tier 10. It will be manually reviewed by their team for copywrite, trademark issues so read the guideline thoroughly before submitting your first designs.
Some people do prefer to start on other platforms because they aren't as strict as Amazon. With Amazon you get one account and if you get too many rejections and get your account closed you normally won't get a second chance. Since Q4 is so close though I would just say apply and be careful with trademarks because the Christmas season will be your best chance to speed through the early tiers.