Hello, newly engaged folx!
Welcome to Wedding Wire - and congrats, you're engaged!

Ok. Deep breaths.
NOW WHAT.
Relax, there's an app for that. (Sorry, couldn't resist.) Actually, there's a LOT of apps for that, so poke around, find which one works for you. (I used both WW and the Knot, because I found they offered different things.)
My first piece of advice is to create an email *specifically* for the wedding. Best if you make it so both you and your FS (Future Spouse) can access it. This will be helpful for planning, coordinating, and filtering All. The. Spam. you are about to get. DH and I did that, and it was the best wedding advice we ever got. Now we use it for certain shared purchases, like mattresses, photo albums, etc.
So, now you have an email, and an app... or two. Hooray! I also recommend a book, but that's a personal decision. We found it a good place to stash all the paperwork and notes for each other. Cool, now what??
Breathe again. There you go. Now you want to figure out how much stuff costs where you want to get married. If you don't know, you could be in for some sticker shock. The cost guide is a super useful tool. It gives you the average costs for your area. (Oooh, hey, plug in New York City! YIKES! Right? Yeah, that was where I had to plan. Now everything looks super affordable, right? ...Sorry if you're also in a big city.)
We have an email, we have apps, we have an idea of how much our region costs.
Now, now you need to get offline, and talk to your FS and those who might be helping you pay, about how much you can spend, and how many people you are inviting. This can be highly charged and emotional, so bring your research, and be gentle with yourself and others. Remember that the truly important thing is to be married at the end of the day.
To help us decide what was best, DH and I came up with what was most important to us about our wedding day. This was our list, feel free to amend.
1. Be legally married by the end of the day!
2. Feed and entertain our guests well.
3. Have a swordfight.
....Should have mentioned we're actors. Sorry.
Everything else after that was either gravy, or unimportant. We ended up putting photography as a gravy splurge.
Right.
We have an email, an app, research into costs, a budget decision, a "what's important" decision, and a guest list decision. Believe it or not, *that's the hard part*.
Now, go take a look at the checklist. (We used both this one and the Knot's.) You can customize it, and let it guide you!
Good luck!
(YOU'RE GONNA GET MARRIED, THAT'S SO EXCITING!)