How to acces worpress database

For the first time I’m trying to create a WordPress plugin.
For this reason I need to open, select and close the WP database because I need to fetch some standard data from it. Of course it’s not so difficult to include the wp_config.php and write my own routine to do this, however I wonder if there is any “official WP-way” to do this. Are there any wp_functions that I can use to open and select the Wp database??

This is probably documented somewhere online, but I’m searchin for hours and I can’t find it.