Make my system as server for ms-SQL

Richard's advice is not about a firewall: it's about the security of your database and the data it contains.

There are two problems here which you need to address:
1) Security: if you directly expose your DB to your customers via the internet, it is accessible to anyone who has a valid username and password - and your customer software that accesses the DB will need both, so that's already a problem. If an employee of any customer leaves under a cloud, you will not know - and if they have the user details they can very easily copy, steal, delete, or damage your databases. Depending on what they contain, that could cost you considerable amounts of money: GDPR fines for inadequately protecting sensitive data are very serious!
2) More security: Did you leave the default sa admin account enabled on MsSql? If you did, anyone with the IP can do anything they like to your installation and the databases it contains.
3) Addressing: do you pay your internet provider extra each month for a static IP address? If you don't, then you almost certainly have a dynamic address - which means it can change at any time, and that would mean your customers would no longer be able to access the DB at all.

All in all, there are some considerable problems that you need to really think about before you go down this route - and you don;t sound like you have enough knowledge at this point to even know what the risks are, much less understand what you need to do to mitigate them.
I'd strongly suggest that you use a hosting service, and provide a web service to intercede on DB access and prevent all direct connections.

Or go for a cloud based DB solution and learn what the risks there are before you do anything else!

Please read Solution 1 and Solution 2 before this one, as they make mention on some good points and then later on read this solution once you want to get started with the deployment of a database.

Quote:

can connect that via internet

This is the only problem in your plan. You can surely host your databases on the machines you own, but the problem comes when you provide a write-access to the customers. And to provide this to anyone on the "internet" is a huge step.

Apart from that, a major problem would be the high-availability for the database. In case your system goes down, for updates, for power outage, for bugs and crashes, everyone that depends on your system will face a downtime in the services. Thus, it is always a bad approach to manage your databases on your own systems. Not to mention, that there are costs associated with every task; electricity, system upgrades, system reboots, data backups etc.

Quote:

can access that

One of the best ways to provide this service is to write an API that exposes the database as a Web API. This way, you can host your web app and the database on the same machine, but your customers can only access the web app—through HTTPS—and your web app then goes ahead and communicates with the database to provide the results.

This will surely require some web app development experience, but I am sure that will be a better investment.

Last but not least, for high-end demands, I highly recommend purchasing a subscription from a cloud-based vendor, such as Microsoft Azure, Alibaba Cloud, AWS or Google Cloud Platform. They would provide you with a database solution with only ~$30 and you scale as you grow. But everything is managed by them, storage, compute, replication, availability and sometimes security is managed by them too.

For cheaper and separate, user-by-user demands, I recommend using local databases on client's machines. Such as SQLite, that can be a good candidate to create a database on the users' machine and write the solution that uses SQLite and provides the solution. But remember, the data is stored on the client-side and is not shared.

SQLite Home Page[^]

In addition to the other suggestions, and especially the one about using an API, you might be interested in:
best-web-frameworks-to-create-a-web-rest-api[^]
best-sql-database-as-a-service-providers[^]
And also: nosql-database-as-a-service-providers[^]