Congratulations!!
First thing you should start with is...
BUDGET.
Seriously, don't even think about trying on dresses until you know how much you have to spend on the wedding!
Some people get financial help offered by parents. I fell into this category, and will be forever grateful for it. However, do be warned: my parents have been extremely reasonable, and only insisted (non directly) on one very small issue (invitations), and not given any other opinions unless directly asked. However, many others on the boards have complained about parents viewing the contribution as a ticket to control the wedding in some way or another.
Figure out which category your family would fall into. If uncertain, make your assumptions clear from the get go.
Second thing - which I had done this way, way earlier actually, is...
GUEST LIST.
I recommend sitting down with your partner, and on separate bits of paper, writing down everyone from your side that you want to come to the wedding. If you both write the same friends down it doesn't matter. But you need an idea of how many people you are going to invite, and to work out how one side won't swamp the other.
I had no idea how many cousins/uncles/aunt's my fiance had before we wrote our lists out. I was like "....who the fuck is this?". You don't need to QUESTION who is being written down, but obviously you need to know numbers, and who they actually are.
Doing the guest list early also means you can contact any very key people you definitely want to be there, and run some dates past them to check they are available.
Leading onto the third/fourth thing, which is.....
DATE. AND/OR VENUE.
I chose my venue first, then picked one of the dates it was available. Be warned that 1st April to 30th September is high season, so everything effectively doubles in price.
This is also why the guest list is so important. One of the key issues about venues, aside from their gothic panelling or 16th century fucking portraits or whatever is......THEIR MAXIMUM CAPACITY.
There's no point looking at a venue which only holds 50 people max, if there are 120 people on your draft guest list.
Finally on the super important things, I would say
BOOKING YOUR NOTICE FOR MARRIAGE.
Or sorting out your banns, depending on where you are marrying. This is the essential thing that actually makes the wedding legal, but you often need to have confirmed the venue before you can book the appointment.