STEP 1: Budget
Everyone wants to immediately jump into booking the venue and other vendors - but first you need to know your budget. This will set the tone for the rest of the planning, and without knowing this in advance - you can't know how many guests you can have, what else you can spend on, etc.
Sit down with those who will be helping pay for the wedding and figure out the total amount you will be spending. If others are helping pay for wedding expenses, find out what the limitations are; are they wanting their money to go to certain things? Are the budgets in place exclusive of certain items? You need to make sure you are very clear on all the details.
The guest count is partially decided by your budget, but initially in the planning you and your significant other need to sit down with your families and figure out (best case scenario) how many people everyone plans to invite. You can call it List A, you can call it a first round - whatever you call it, just know it will change. Some will be added to the list and some will come off the list. You just need a starting point.
Step 3: Hiring a Planner
(ok, do you blame us for putting this on the list?)
But seriously, no matter who you hire - you should hire a planner. Whether they are helping you entirely, or closer to time - you need a professional who can help guide the process. You need someone to help format a budget, and give you realistic expectations of expenses (don't believe everything you read on all the wedding platform websites). You need someone who can help review contracts, make vendor and venue suggestions, and who can keep you from booking and/or paying for the wrong things. You'd be surprised how quickly a budget can be blown out of the water!
The fourth step is finding your venue. Your budget and guest count will come in to play here - if you need more room for your larger guest count, but need to bring in your own caterer to help reduce the costs. Or if you have a smaller guest count but want a certain feel and decor for the reception. There are TONS of options and things to consider when looking for the perfect venue, so don't jump into anything. Plus, depending on where you go things like catering, bar service, cake, decor, rental, floral, etc may be included in the price.