For starters, when you signed up and created your SMS Central account, we gave you 10 free SMS credits for you to test.
When you go about creating your SMS campaign, in Contacts (step 2) select "I want to add contacts manually" and enter your mobile number (or any other number(s) who would like to see the campaign). Make sure there are no spaces between numbers.
NOTE: If it's your first time EVER, your "All Contacts" group will have your own mobile number, so you can just go ahead and select it.
Keep going through the wizard (compose your message and select send now) and you will receive the campaign on your mobile phone.
If you'd like to use the same message later for another campaign, make sure you save the message as a template so you don't have to re-type the content.