Yes, definitely finish first (that's an achievement in itself!). Then set it aside for at least a month before going back to edit it.
It's worth going to one of the writing festivals, if you can afford to. There's one in Winchester in June and York in September. There are lots of useful workshops, you can meet other writers and also get a 1-2-1 meeting with an editor or agent to look at your work.
I'd strongly advise finishing and thoroughly polishing before submitting. I was badly caught out by submitting when half way through and getting a next day request for my full ms! Thorough preparation essential!
I've always loved writing; have entered a few competitions over the years and had the odd short story or poem do well, but have never looked at doing an actual book. However, I've had an idea brewing for a while and have finally done a full synopsis, character list etc., and have started writing
Once it's finished (note the optimism here! Assuming it gets finished...), what do I do then? Do I submit to agents? How do I know which agents to submit to? Do I need the whole book finished before I start looking at agents? Also, who do I get to proof read it?