It is a real matter of choice.
Your basic tactical squad will probably be present.
Sternguard are great in the new codex. All the Hqs are pretty solid now with the librarian probably being the best overall.
I would recomend gettinga ssault on black reach if you can find a copy. It is a captain, a tac squad, 5 terminators, and a dreadnought as well as quite a few orks for a severely discounted 60 dollars. Its a great start to marines.