Try to use what you have.
For the start, you can do planetary and Moon photography - you only need to attach your camera with suitable adapter, for example
this one.
(you may need to move your main mirror a bit to achieve focus).
Also, you may want to use barlow lens with this setup (like
this one) as well, to increase the size of your image on sensor.
For deep sky, you need
tracking unit for your Astromaster, then you can use your camera for longer and multiple exposures (for stacking).
DeepSkyStacker is a good choice for software, but first you try above.
There is always a time for upgrade (and spend more money) later, when you get familiar with equipment and process..