Pagman, I would recommend breaking this learning process into two pieces.
You've got hardware, and you've got post processing, both of which can pose challenges for the beginner. I would put the hardware off for now and focus just on the processing.
First, put a wide angle lens on your camera so that you don't need a tracking mount. You can take relatively long shots without getting star trails if you have a wide angle lens. Take several such shots, take darks, etc. and then learn how to stack shots, deal with noise and light pollution, etc. The advantages of doing this is that a) you don't have to muck around with expensive mounts, learning how to polar align, etc. and just stick with your tripod, and b) you don't really need to find your way around the sky, which can be a learning process in itself, unless you are an experienced visual stargazer, in which case this does not apply to you. All you've got to do is to find a relatively dark place, point the camera at a relatively rich part of the sky and shoot away.
Once you've figured out how to do the processing and are getting good at taking shots of star fields, then you can focus on getting a better mount so that you can take shots of deep sky objects with your 300mm. You also would need to get reasonably good at finding things in the sky (or you can get a go-to mount, in which case you basically punch in the name or Messier/NGC number of your object and the mount does the finding for you).