So random question- does anyone know anything about how to create a portfolio?

At the moment I am trying to put myself through a home made art & design course to practice my current skills and learn a few new ones. It is fun, but it is very slow going and I only have myself as a judge of my work. If I can afford it then in 18 months or so I'd like to sign up to do a Level 3 Certificate in Foundation Studies in Art & Design. I'd gain more experience, get feedback from tutors and even be able to exhibit some of my work towards the end of the course. 

The only trouble is to land a place on the course I need to present a portfolio of my previous art & design work, and I've never made a portfolio before or even been taught how to create one. I've spent several days trying to research it myself but most websites talk about portfolios for landing spaces in galleries rather than gaining entry to study courses. All I've really gathered is that I need to have 15 to 20 pictures of my work and that I need to spend a lot of time making sure it is the very best it can be.....but that leaves most of my questions unanswered.  

Does it need to be a certain size, ie: A3 or A4?

Is there a specific format that needs to be used?

Do I need to buy a specially designed portfolio book to use?

Do I present photos of my work or originals?

Do I just make a collection of stand alone images or does each piece need a written description explaining the idea behind it?

Some websites mention sketch books, do I need an accompanying sketch book for each piece? If so what's the expected number of pages for each book?

Do I need to write a covering letter or a personal statement of some sort?

Are portfolios for study meant to be "artsy" and creative in appearance or strictly professional looking?

Gah! Why didn't any of my art teachers at school talk about this stuff 8 years ago?

Any help or advice will be very gratefully received!