Hi there,
You do not need to buy at this first time, assuming you do not have graphic design experience you can start by using an Apple application as Pages, Keynote, iPhoto or others in your /Applications/ folder, check it out, try opening applications, read tutorials and welcome presentations or open a template file for some start edition.
After that you can try a freeware third party application, you will find many of that here:
http://osx.iusethis.com/tag/graphics
Professional applications require a minimum specific knowledge that you would naturally acquire after sometime around with these others.
Before go on using third party apps would be smart to get one smart removal tool so that you'd be able to remove applications you do not like and related support files in one move, which would be better than having useless files remaining in your system. I use AppCleaner -
http://www.freemacsoft.net/ - with it's SmartDelete Preference enabled, set it up if you will.
Despite most of Mac's applications only need to be copied to your HDD to be ready to run, they usually keep some support file maybe elsewhere the ~/Library and/or /Library/Application Support/ folders.
Good luck
