Looking for affordable dslr?

I'm into photography, and I want to get better at it. Currently, I have a Nikon L100, and it isn't very good. Though it has 15x zoom, the focus is terrible, and nightime, or low light photos, along with high light photos, and really bad. I'm looking for something affordable, preferably under 600 bucks. Any suggestions? Please suggest makes and models.

You can move into the Nikon D3100 for about $500 if a DSLR is what you want.
Otherwise I'd suggest the Nikon P500 which is a bridge camera. It can be used as a point and shoot or fully manual-cannot do that on your L100.It has a built in zoom that goes from wide angle to mega zoom.
If you want to really get into the meat of photography and intend to build a kit of lenses and accessories: go with the D3100.
If you want to really be able to manipulate the photo, but don't want to end up with a whole kit of lenses and assorted stuff that will cost you, then go with the P500.

Nikon D3100

The Nikon D3100 and Canon T3 would be your best bets for a DSLR in that price range. There's a buying guide on DSLRs here: http://www.learningcameras.com/shopping-guide

I would try to pair that with a good lens for low light use such as a 50mm f/1.8 which is only about $100 and will give great results in low light. A DSLR paired with a lens like that will give you great results due to the large image sensor and large max aperture, both which contribute to great low light performance.