Hi,
I agree with Scott on some of his points, i think it will be wise to have a goal for your portfolio as well.
to put things into perspective a bit,
let's say that you like to create a portfolio in order to apply to a 3d Visualization Studio.
Research
First of all, you need to do some research and have a list of studios ( that you'd be so happy if they hire you).
This is very important, as it will give you a broad scope about what is it that, you should include in your portfolio, so that they get interested to hire you or do business with you.
you should be clear about that, brainstorm all the information about what they do, in what kind of sectors they are in, which country etc...
This not only will help you get ideas about what kind of work to create in your portfolio, but also give you the ability to talk with them with a language they understand.
when you show studios work that is similar to theirs, quality and genre, they are more likely to to listen to you.
Expertise
Second, it's okey to use free 3d models, the most famous 3d visualizers do, it doesnt mean you don't know how to model, you just need to do some few things to convey the HR, that you have modeling skills, and yes i know some visualizer that have only one task, lighting and shading, they don't do any modeling!
so how you can tell you future employer that you got the skill?
here is some ideas,
1 - Tutorials.
Making training about the skill.
2 - guest post ( making of).
For one of your works, you can add it to your portfolio as well, it will get them an idea about your workflow.
3 - social endorsement.
Use linkedin, facebook, forums etc..
things like that
Hope this will get you some ideas to start.
Best,
- Ismail