You can create segments in the 2nd step of the journey builder and will soon also be able to create them in a segments tab under audience

In the journey builder the segments tab is the 2nd step: Example Flow

If you wish to include all users, you can select all users: Example Flow

To create a segment select “When customer meets conditions” Example Flow

Using a Segment in a Journey

Once you are happy with the segment definition, you can review the segment in the final step on the journey builder and before setting it live