To start using MySQL, run
/opt/lampp/bin/mysql -u user -p, where user is your user for MySQL (you can use root).
You need to make sure that xampp is started (if you start the whole of xampp, then mysql will start with it). Otherwise, start mysql by typing
/opt/lampp/lampp start
Then you will have a prompt like this (or similar)
mysql>
For tutorials on MySQL, see
www.mysql.org.
Here's a tip:
If you're already in /opt/lampp and want to start lampp from there, instead of typing
/opt/lampp/lampp start, you might want to type
lampp start directly, but you might have to type
./lampp start. I'm saying that because it took me ages to figure this one out...