What should a good beginner copywriting portfolio include?

Read "Obvious Adams" - it's in the Public Domain so you can
download it. That should give you some inspiration. Also
the "Robert Collier Letter Book" and Claude Hopkins'
"Scientific Advertising" discuss the early careers of those
gentlemen. It might seem corny but I read one book by
one of the greats where he wrote and laid-out ads on his
own time for months and then showed them to a superior
and they gave him a shot.

Others will probably chime in with different advice, but I will instead suggest what to avoid. These are tipoffs you are a beginner, and don't have a clue.

...Focus on a specialty. Take inventory of what your strong suit is, and produce writing examples for that. Newbies reason that they should do health, diet, B2B, consumer products, IM, nonprofit, public relation, print, web, radio, catalog, TV and MLM examples.

Scattershot portfolios, hoping you'll hit a customer is typical for the beginner. Try to avoid it by understanding and playing to your strengths is the best way for a beginner to produce good results for a client. And, by producing good results for a client, you're more likely to develop good word-of-mouth and testimonials.

...Avoid cliché and lemming-like writing. Like that "Who else wants..." headline? So did 237,564 other would-be copywriters. Understand you don't write even mediocre copy in a competitive vacuum and you'll be ahead of the vast majority of beginner writers.

The transition to a creative within the agency environment is a tougher question. This includes politics, personalities, and type of agency.

For example, if your agency uses the word "creative" they might be exclusively image advertisers -- and shun direct response. Trying to promote direct response to this crowd would be the worst thing from a transition for your current position. (As a side note, learn about the different philosophies of advertising, like image and direct response).

Alternatively, if you know the agency has missed out on (or lost clients) because they're all image advertising -- then take advantage. Some agencies are proud they don't lower themselves to "base commerce," others hate losing those clients, or the agency might already be exclusively direct response.

The point is your success is going to hinge on understanding the culture and politics unique to that firm, and demonstrating that knowledge in your proposal for transition.

Finally, either way, understand how copywriting and creative production fit in with marketing. Understand what your role is and is not.