I'm assuming you write fiction (it's different for non-fiction I believe). If so then I wouldn't submit to an agent until you are 100% happy with it and finished. If an agent likes it and asks for the full manuscript you need to be in a position to send it. Plus, they like to see that you are able to finish a manuscript!
When you are ready - most will ask for a synopsis, (usually a short one so an A4 page) the first 3 chapters and a bio. (Check what each agent wants as they are sometimes very specific and if you don't give them exactly what they want they won't even look at it.)
As most agents, now ask for submission by email - they will ask you to put the title in the email, together with the genre and an elevator pitch. This is exactly what it sounds like - a one or two liner that summarises the book. That's a tough one but some agents won't even open the email if the elevator pitch doesn't float their boat.
Your bio should contain anything good in terms of your writing - eg competitions won, etc.