to start with - what printer did you buy ?
ctc make a lot of different models.
And depending on the motherboard - it can make a lot of difference.

secondly, you need to find out what com port the printer is using and what connection settings it's using. Then tell rep-g so it can talk to the printer.
rep-g with slic3r is about as easy to use as it gets in the free sector. ie: rep-g can also be used as a slicer.