I'm a newbie and trying to build an automated solution to navigate inside a warehouse with a drone, using virtual sticks.  Without GPS, how can I orient and navigate the drone around accurately? Could I get any advice for this?

I think you may need other sensors to determine the position of the aircraft. Such as lidar or vision. Maybe onboard SDK is more suitable to achieve your request.

In theory you can use velocity and control theory to calculate the distance traveled by your drone with APIs that read and control velocity and time but this technique is really prone to errors.  

What I've seen many developers do instead is use a drone's vision systems to read pre-set markers (like QR codes) laid out in the environment to orient and move the drone about - what this means is, you will have had to have scoped out the environment prior to flight of the drone and design navigational markers using other tools.  But if you can do this then you can use many of our drones in this scenario, by accessing videofeeder with the MSDK to read and interpret the markers and then feed instructions to virtual sticks.

Or, like the previous reply from kv886 - you can attach special equipment to a Matrice series drone and use the OSDK.  Lasers, vision, lidar, sonar etc....

Please note, that the P4P has a sonic feature, which allows you to use this API: getUltrasonicHeightInMeters allowing you to calculate vertical distance within a certain range (I believe 15 ft).  In addition you can also use getTakeoffLocationAltitude and getAltitude to calculate vertical range, however both rely heavily on barometric pressure which is very susceptible to changes in the environment - for example if someone opens a door in the ware house, that changes the pressure in the entire environment and therefore your readings.

This is a challenging thing to accomplish - so keep trying, get creative but just know, that flying in non-GPS environments is complex.