You've conquered all the other species in the immediate area and shown the world your dominance of all its inhabitants. You aren't quite there yet, though. A species must come to terms with political divisions within itself before it can make it into space, and that's exactly what you aim to do. Whether by propaganda, force, or just buying out any dissenters, you must unite before you can venture into the depths of space. So get ready to dig into Spore's Civilization Stage!
As you make your swift exit out of Tribal stage, you should be given a chance to change your species outfit. Take it. You can change it at a later date, but setting up your outfit now should save you time later. You will also be prompted to create your city hall (of which a copy will be at the center of every city you own) and create your ground vehicle. Much like your creatures, they can look like anything you want, so have fun building them! Upon finishing your land vehicle, you will start the Civilization mode proper.