You probably want one with a key. The two best known are
North-Hillard Greek Prose Composition, which starts with simple sentences which are organized according to structure.
Sidgewick's "A First Greek Writer" has many lengthy explanations and examples. But the composition is in paragraph format (which is good) but a little harder.
Both books have keys and recommended vocabulary. Either way you should read the associated grammar sections in each.
There is an ongoing group with the GreekStudy list which is using North and Hillard. They are on lesson 8 this week. You can find more about it at http://www.letsreadgreek.org/moodle2
. You can even do your own and then look back at the submissions by others for their errors and yours.
The best way to write is to start doing it and discuss the now and then. There is a small group on the GreekStudy email list (you can Google it) called Dialogos. On it people send emails to each other in Greek (Attic or Koine). There is also a Ning site (at least for now pending Ning's elimination of free websites and changes) where people discuss in ancient Greek. The site is still under development, but it is usable. It can be found at http://schole.ning.com
Hope this helps.