I did that when first started too!
I'm not sure how much you need to know...so I'll walk you through the process. First you have to start XAMPP. You want to click on the orange icon...called XAMPP Control. It should be in your applications folder. If you can't find it, do a spotlight search for "Xampp Control" and click on it from there. A small grey box will come up and prompt you to start Apache, MySQL and FTP. Start them. Also along with the grey box, a white text box also came up. It's telling you that you either set up XAMPP through http://localhost
or the default sites folder. I'm guessing you have it set up through localhost? If that's the case then open up your browser and type in: http://localhost/wordpress/wp-admin/
That should bring up the Wordpress login screen.
Note: I'm taking for granted that you set up your wordpress installation under a folder called 'wordpress'.
Hope this helps!