Baically I want to add a simple trading system in monogame using SQL but I’m not sure how to
Is your question how to connect to a DB server and run SQL queries in C#? If so, you can use .NET’s System.Data.SqlClient.
If you’re asking more holistically: you would setup a DB server (this can be as simple as a VPS or any other basic low-cost web host running some flavor of SQL server like MariaDB). You would then use a tool like HeidiSQL (or even just phpmyadmin if your host has cpanel) to setup your DB structure (tables, columns, etc). You would then setup the connection/queries necessary in your game to insert/search/etc that DB using System.Data.SqlClient.