If you own a game package, you can play. There is a player cap on each instance of the 2.0 universe (Im not sure the exact number though). Enough are playing that sometimes I get the "All Instances Full" message when trying to log in, but once you get into your ship and are out screwing around running into others isnt a real common occurrence (at least for me).
The Javelin is probably a bit off from being flight ready. In the Star Citizen
Ship Matrixyou can see a description of all ships that have been revealed and sold at some point so far. Each entry has a status update that shows where in the pipeline it is. The Javelin is in the 3D art stage, so it's still a ways off from being Hangar ready, much less flight ready. Currently the largest ships that are flight ready are the
Retaliatorand the Andromeda variant of the
Constellation.