You can start now. I started planning almost two years before and I wish I had more time. You can start looking at venues. You’ll have more time to pay of off especially if you’re having a bigger wedding and you’ll have more time to buy decor invitations and ensuring you have the date you want, popular photographers, djs, florist, hair and make up artists. You’ll literally have your pick of everything. This gives you the opportunity to have the wedding you dreamed of because you have the time to save for everything and purchase all the little things because they’re are SO MANY things your going to have to to make your special day unique and exactly to your taste. CONGRATULATIONS!!!!
12-15 months prior to your wedding date should be sufficient. Choose your venue first, to ensure they have your desired date open. They may have an on-site wedding planner and/or preferred vendors they recommend, which can streamline the process greatly.
We started planning about 2 years in advance! We wanted a specific date and weren't willing to budge on it so we wanted to make sure our venues, at minimum, were booked when their 2020 openings were available! They came available Spring of 2018 so we decided to book then!
For about a year we mostly just did fine tuning of what we wanted the "feel" of our wedding to be like! We decided we wanted elegant mixed with slightly casual (ie: no full suits except FH, lots of babies breath and roses and eucalyptus, candles, etc.) which led us to our wedding colors of wine/burgundy, gray, white, and black.
You could definitely start now! I got engaged last year and we are starting now to put deposits down for our vendors for my first year I just did a bunch of research and venue tours to see of where I envisioned of where we wanted to get married, but you definitely got plenty of time to plan. In all honesty I would enjoy your engagement and soak it all in before you start making big decisions.