The venue and date is the thing to sort first. For that you need to know roughly how many people you want to invite and your budget.
You work out if you want traditional type church/estate wedding or something else, beach/woods/parachute wedding.
Ideally you would book the venue, at the vary least, 1 year in advance but 18months/two years might because for popular places.
Once you have set a date, you need to organise food, drink, seating.
Once venue and food is set up. Send save the date cards to guests.
Then clothes for bride and groom plus bridal parties if you have them.
Then hotel room booking if away from home. Plus shuttle buses for guests if needed.
These are the really important stuff to sort everything else is just frills. I would recommend a photographer personally as very worthwhile.