Today a person reached out to me on Messenger, explaining that she was her son's "Momager" and assured me her son could do any type of project I had to offer.  She showed to me a couple of nicely-drawn sketches -- unfinished pieces -- and I asked her to send to me his professional portfolio.  He didn't have one, so she told me she would assemble a .pdf to email to me.
Her son sent it a few minutes later, labeled "portfolio.pdf" rather than including his name.  It was drawn well enough but was a typical "school portfolio," or more like scans from a sketchbook, with zero finished pieces and zero attempts to replicate any kinds of covers, logo, magazine or design work, character or toy design, or any of the usual things one sees in professional portfolios.
So I responded --
Thanks for sending along your portfolio.  It confirms what I had guessed based on the examples your Mom showed to me.  You have a lot of raw talent, but your portfolio does nothing to show you can do professional work for clients.
A perfect portfolio looks like you've been working as a professional artist for years, even if you're just starting out.  
The easiest way to figure out what you need to include is to put yourself in the shoes of the editors or art directors who will review your work.
If you're submitting to a client who is looking for cover art, then you need to show some cover art.  for example, if an artist submitted this, I'd know he's hirable --
There's a sense of design, some humor/whimsy, and flexibility of style, ass well as varied subject matter.
If you're going to offer things like logos/corporate identity or brochure designs, we need to see those --
If you're going to offer, for example, being able to draw in licensed art styles, of course you need to show it --
Simply put:  If you're looking for professional work, your portfolio has to prove you can do it.  Potential clients won't take your word for it.  The won't take a family member's word for it.  They have to see you've done something like it.  
What's more, a portfolio becomes a growing, constantly improving thing; after you get your first few jobs, you'll find yourself adding your best new stuff and dropping your older, less impressive work.
One last thing:  If you build an online portfolio, be careful NOT to make it into a storehouse or depository for all your old drawings and sketches and finished pieces.  No potential client wants to wade through all your old stuff -- and any inferior/earleir art just weighs down the overall impression of your talents.
All my best to you,

-- David