You can start your application with Boundless even before you've married your partner.
In the meantime, you can start gathering supporting documents for your application. By completing this step before you marry, you'll be ahead in the process by the time you're ready to file with the government. Until then, your Boundless account will allow you to keep all of your information and documents securely in one place.
Here's a detailed list of the most common supporting documents required for a marriage-based green card.
And this guide can help you track down hard-to-find documents.