Your Default login information is sent to you via Email once you have purchased a server and it becomes active. It should be in your inbox, but if you do not see it, check your SPAM/JUNK folder. Click the link in the email for your control panel, then log in with the information provided. You can change your password following the guide at this link: https://ggservers.com/billing/knowledgebase/63/Change-Your-Multicraft-Password.html
If you are unable to locate your welcome email, you may click "Hello, NAME" at the top right hand corner of the website after logging into your account and choose Email History. This link will provide you with your entire Email History with GGServers. The email would be title "Your Minecraft Server is Ready."