According to me designcrowd is the best site for beginners freelancer. Because compatition is low here and aslo get participation payment sometimes when you are not winning main price. I also earned $20k on this site.
You can aslo work on:
99designs
Talenthouse
Crowdspring
Designcontest
Peopleperhour
Upwork
Fiverr
For more info about freelancing and graphic design follow me on YouTube www.youtube.com/c/kasturilal
Thanks