So, more often than I care to list, I hear from students and people wanting to make portfolios that they don't know *what* to work on, or that they have a hard time focusing and making finished work.
One great tool for this is to find online challenges. Many of the art websites out there do either semi-regular, or even weekly/monthly challenges. Brainstorm, Artstation, CGSociety...they all have these.
Sometimes they are broken out by "character", "environment", "concept", "production art", "matte painting" or something similar, and sometimes they are just topics.
These things are great. They give you a deadline, they give you a topic, and then give you a built in audience to give you crits and feedback...in some cases, they'll even give you a job. ILM hired several people based on their Star Wars challenge last Summer. They also show you the range of talent out there. Some entries are obviously done by pros, and others by bare beginners....which brings me to my next point:
You should do these even if you suck.
First off, no one "sucks", they are just not as far along the path...and these things are like going to the gym instead of working out in your living room. You may feel self-conscious that you can't bench your body weight, or that your gut kinda hangs out when you do squats...but no one really cares. Everyone is there to push themselves and to get their own thing out of it, and if someone picks on you for your level, they are the ones that come across as assholes, not you. Do the best you can, post your in-progress work, and try to learn from other people who are trying to solve the same challenge problems that you are.
That being said - If you are more of a noob, you may not get a thousand 'likes' or comments. Often, people don't really know what to say if your work isn't up to a certain point...they don't want to hurt your feelings, but it's hard to offer crit if you are posting thumbnails and calling it finished work...remember that the people in the challenge aren't there to help you, they are doing their own thing as well.
Like I said, these things are everywhere, but currently, Artstation.com is doing a whole series of these based around "Ancient Civilizations: Lost and Found" I've signed up for both the keyframe and environment design categories. There's a month until the due date, please feel free to follow along with me as I post thumbnails, sketches and other in-progress stuff.
I hope you join me on this one! It's not about winning, it's just a tool to focus your energies.
Thanks for reading :)