The first thing you want to do is discuss with your partner what their wedding vision is, and work out a timeline. Do you want to have an elaborate affair with lots of friends and family? If your answer is yes, you're gonna need to take some serious time to get it all together (not just the planning, but in all likelihood you're gonna need to get some funds together, too). Most engagements nowadays are at least a year and a half in length. Some even stretch out to 2 years or more. And THAT'S OKAY! You're planning on spending your lives together, right? Take some time to plan out your big celebration and do it up right! Even if you're not planning a large fairytale wedding, taking a year or more to just be engaged is highly advisable. My fiance proposed just before Christmas 2018. Neither of us wants a big ceremony, so in all honesty we could go right down to the court house right now and grab a license, ask a friend to sign it (I know a few other officiants who'd love to), and be done with the whole thing. BUT!!! Just because we don't want a big ceremony with lots of people doesn't mean we're ready to jump into marriage right this instant. We DO want a small ceremony with our kids and immediate family. We DO want to celebrate and have dinner with them. We DO want to then spend the weekend away, relaxing. And even just those small things require money and time to prepare. So we set our date for August 2020. Just over 18 months should be plenty of time, and it works for both of us.
Take your time and pick a date that makes sense to you, and keep in mind that some dates are much more in demand than others, when it comes to vendors and guests' availability. Once again, the more lead time you have, the better. Vendors typically book up to two years in advance. It's May 2019 now, and I already have bookings as far out as November 2020. Keep that in mind. In this area, May and October are the busiest two months for weddings. If you want a wedding during one of those months, chances are you're going to need to look into booking some of your vendors (and your venue!) pretty early on. With booking comes deposits. Deposits cost money. If you have to change your date for some reason, you might end up losing your deposits, so consider your date very carefully.
Next, you're going to want to figure out a rough headcount of guests you'd like to invite BEFORE you put down a deposit on a venue. You don't want to book a small-ish venue and then end up finding out there won't be enough seating for all of your guests. That could result in some hurt feelings when you realize you're going to have to pare down your guest list.