In order to get into the art portal you have to get scouted by another artist. Just start off posting some of your art (at least 4 pieces I think) to be eligible and to get noticed. Just be sure to follow the guidelines though. The links below are the art guidelines and in the FAQ scroll down to the "Art Portal Questions" for more info.
Best of luck!
https://www.newgrounds.com/wiki/help-information/terms-of-use/art-guidelines
https://www.newgrounds.com/wiki/frequently-asked-questions
AmmoGuppy
Hello. Welcome to Newgrounds! Your best bet to join the Art portal is to be scouted. The link listed below is an FaQ made by Ornery that will help and explain other things in that process. Best wishes!
https://ornery.newgrounds.com/news/post/388624