If you plan to submit to agents then do that first. If you submit your manuscript to publishers and are turned down by all of them, and then think, oh, I'll get an agent--you have just limited the number of editors your agent can submit a shopped manuscript to. They are not going to bring it back to someone who has already turned it down.
Yes, it is difficult to get an agent. But it is difficult to get an editor, too. And many editors will not even look at a manuscript unless it comes in via an agent.
If you aren't interested in an agent then you might want to go to conferences, as sometimes visiting editors will allow you to submit things to them during a short window of time after the conference (even if they are normally closed to non-agented submissions.)
Anyway, just some things to consider.