First off: This is nothing but MY opinion.
Since you only just started, I don't expect you to do mixdown masterpieces. I think what's really important right now, is that you are getting comfortable with the DAW of your choice.
Try creating some really unique sounds, that you had in your head and put together some songs first. No one starts off and just learns it over night.
I think your sennheisers are more than enough for that, but maybe don't use the built in speakers, they truly are nothing of value ;).
Once you feel ready for the next step, get yourself some sweet headphones (preferably two pairs with two entirely different characteristics) and some nearfield speakers for reverb mixing.
That's probably the cheapest way to go for a good monitoring system. Once your track sounds good on all 3 systems, your mix is on a good way.
See, once you get deeper into this entire music production/engineering and so on thing, you'll notice that you could literally spend THOUSANDS of dollars. Get comfortable with your gear and you'll succeed eventually. Because the best gear isn't of any use if you don't know what to do with it. :)