First step always is creating your budget-- if you don't know what you have to spend, you'll end up spending too much, and that applies in all areas of life! Sometimes this takes more than a few minutes because you will ALWAYS forget something! "What's that you say? We need something for people to toast with? Can't they just do that with air?"
Secondly is creating your "look"-- do you want it to be a simple spring wedding or are you looking for the pretty pretty princess wedding with jewels, a big dress, and a cake taller than yourself? Either way, you'll want time to make the centerpieces made from Bell jars OR to save up a little extra coin to rent out the Kennedy family jewels and find a baker who does the best combination of almond cake with raspberry filling.
Thirdly is dress shopping, for you AND your wedding party-- since we gave my bridesmaids a lot of notice, they were able to find their dresses on sale which saved a few of them almost $50!! For a volunteer position that requires such a financial commitment (especially on the part of out of towners), it's not just nice but polite to give your wedding party as much forewarning of possible.
There are a thousand other little details to think about with your wedding and there won't ever be a time when you have everything covered, which is why my profession exists! If you can't afford to hire a wedding planner, I've got you covered-- once our move is finished I will start producing my own DIY planning handbooks to be sold on Etsy! (don't you worry, I'll keep you posted on that)
All of that said, the biggest tip I could ever give you is... write it all down! Take a little note pad with you and jot down every idea that comes in to your head from the second you start thinking you might get engaged-- when you have a date set, you can finally start working everything in to your calendar!