Written my first book. Now what?

Definitely, definitely get an agent. Trying to navigate the world of publishing without one is possible, but it's a bit like representing yourself in court - only the professionals know all the rules and what not to do. You could waste a lot of time and be completely discouraged before you hit the right publisher. Never pay out anything - unless you simply want a few copies of your book for you and your friends with no subsequent income from your efforts. Fees are all about commission - which is taken from the subsequent sales of your book.