upload them to a DB on internet
There are no databases on the internet. There are databases on servers that are also webservers. Some of those web servers serve up pages that contain forms whose methods invoke scripts that manage the database connection.
For the moment I would use xampp for DB.
xamp is a package that contains a database tool. It is not a database by itself.
I think maybe it is client mode, because I already have a server (xampp or other in future).
A server serves up data. Your Arduino is not serving up the data. It is a client. It is asking the server to provide a reply to a GET or POST command. That GET or POST command identifies what script on the server to execute to process the command, and what data to use to process that command, if any. In your case, the script to execute would expect some data (what to store in the database, for instance). It would generate a response that defined the success/failure of the command, among other things.
I think I willl upload data to DB using php or sql.
Your GET command will invoke a the PHP script that interacts with the database, use sql commands.
I don't know what is the easiest way using arduino.
Your issue isn't with the Arduino as the issuer of the GET command. It is with understanding the client/server relationship, with setting up the server, with constructing the database, with writing the PHP scripts, etc.
Forget about the Arduino for a while. You need to do all the server setup, setting up the database, the database accounts and permissions, the PHP configuration, the PHP script development, etc. and test it all from the same PC. Once the server is configured, test it from another PC. When it all works, get the Arduino back out of the drawer, and make it call the server with the data that it has.
The learning curve is pretty steep. The feeling of accomplishment when the Arduino connects to the server ans successfully stores data is great.