I need the final steps for shopify. I have all the products and need to start working with my own domain .cl and integrated mercadopago

I have a Shopify account already set up and need help with the final steps of setting up my store. I need assistance making minor design changes so my store looks just right. I also need guidance on marketing and SEO but I am planning on handling this task myself. I need help with integrating my domain .cl and payment gateway with my Shopify account so everything is connected and up and running. If you are an expert in Shopify, I would really appreciate the help and look forward to hearing from you. Thank you!