Celebrate. You (and maybe others as well if you already have a team) have worked hard to get this to point: time to recognise that, and acknowledge key contributors. Seriously, take a rest for a moment and raise a glass. Also take a moment to reflect on everything you've done so far: what went well; what you could improve as you go forward.
Start your venture, put that marketing plan into action and be awesome. You've come too far to turn back. Think about specific launch messages or announcements you can post on social media, colleagues and contacts you can share your website link with, and so on. Get the word out there. Consider a particular launch event, or maybe a series of events, where to hold them, and who to invite.
Enjoy the adventure. After all, you're the boss now, right? There will be highs and lows, but the rewards are worth it - captain of your soul, master of your own destiny.