it's a bit of minefeild to be honest. what you need to do is finish the book. and edit until it is perfect.
after that you need to get a copy of the writers hand book by barry turner - it lists all the agents and publishers in the uk, us and ireland.
along with the contact details you will find whether they accept unsolicited ms or not, and what sort of material they are looking for.
then you send them a query, some ask for just a query letter, others ask of a query plus a synopsis, some want the first three chapters.
make sure to follow the guidelines to the word because a lot of agents and publishers won't even look at your work if it isn't submitted the right way.
get a copy of 'from pitch to publication' by carole blake. she runs a literary agency and rreally knows what she is talking about. it's the best book on publishing i've read, and it really lays it out and makes it simple.
sometimes they will just reject the work, sometimes they will ask to read more or it. sometimes they will pass on that too.,
it can take years and hundreds of rejections before you get published. it doesn't mean you can't write, it just means that you haven't found the right place for your book yet.
don't loose heart.
but the main thing is to finsih first