Hi, a few things--
First off, those pics aren't bad for a start. Obviously you want to get some more stars in, but you're definitely on your way.
I had a look at one, it looks like you shot it at 18mm, f5.8, ISO1600? If so, bear in mind you want to open up the aperture as much as possible if you're trying to capture as many stars as you can, as quickly as you can, without trailing. If your 18mm goes to f3.5 (I assume?), shoot at that aperture. Even f3.5, though, isn't all that fast of a lens--you'd have to do longer exposures.
You might want to try using your 50mm f1.8. That speed will at least gives you a fighting chance at getting lots of stars on the sensor quickly. Obviously you won't have as wide a field, but it'll give you a chance to do some comparisons with your 18mm lens.
Second, shoot photos in RAW rather than jpg if you aren't already. RAW will allow for more post-processing.
Third, you'll certainly need a clear sky (no clouds/haze), otherwise even the best photographers will be out of luck trying to shoot star pictures 
But really, there's no huge trick to it--set your ISO as high as you can without lots of noise (start at 1600), open your lens aperture as much as you can (use Manual mode or Aperture Priority to do this), put your camera on a tripod, and shoot for as long an exposure as you can (how long? Answer: try bunches of different ones and find out!)
Then, use some post-processing as needed.
Good luck.