the next step is finishing the first draft. then you put it away for at least a couple weeks, and then you begin the revision process, which will probably take longer than writing the first draft. then you give it to a couple of discerning readers (ie, not your mom) and really listen to their feedback. then you do more revisions. then you do another round of revisions.
THEN you start to think about agents. and yeah, you want an agent, unless you want to self-publish or go with a very small, independent press. almost none of the major publishing houses take submissions without an agent. so you do a ton of agent research, reading blogs to find agents who represent what you've written. then you go about crafting a perfect query letter (which is, in itself a task that requires a whole lot of revisions).
you might want to hang around the message boards at absolutewrite.com to get some more feel for the process and do some agent research. or, the book The Forest for the Trees by Betsy Lerner is a great book about the publishing business.