Zoe,
You aren't alone in your quest. It is however quite a large frontier and does vary from country to country. In that respect, I'm reserved not to write a long answer with a US slant, if you aren't in the US. Know what I mean?
In general, I think it is important to have an ISBN and bar code on your book. The ISBNs are sold in lots of 10 and the numbers are coded specific per publisher of record.
Nadine