Hi Anna. You have to first create a group. If you don't know how to make one, check page 104 of the user manual. You can get the manual from the Samsung Galaxy Centura page.
Once you've setup your group, go to Contacts, tap on the Group tab then on the group you want to send a message to. Tap on Menu and choose Send group message.
How do you send a group message on a Galaxy Centura?
There is a possibility to send a message to a certain group just by typing "Group Name" in the recipient field, and it will send an individual text to every person in that group...
...but starting from Android 4.1 that opportunity seems to be missing...
as an option you can install and use Contacts GroupU - a free app that has required functionality... here is more details: Google Play.