Well there are a number of things to cause bad quality prints…
First thing you could do - post a photo of an example print. If we can see it, we can point out possible causes and remedies.
As a general helpful guide, have a read of this:
If you have just built a K8400 kit, then the most common newbie issues are:
• Poor x-y axis alignment. See http://www.k8xxx-3dprinters.crimed.be/w/index.php?title=XY-Carriage_Alignment which explains the issue better than the Velleman manual!
• Poor bed levelling - Use a piece of A4 paper folded twice as a feeler gauge between all three points of the bed - the paper needs to have slight resistance between the bed and the nozzle.
• Incorrect print temperature. This depends on the brand of ABS being printed. Poor layer adhesion - print hotter. Mushy blobby parts, print cooler. Make a 5C change, see what happens.
• Use a Brim when printing ABS without a heated bed. Avoid large parts. Try to keep the print area / chamber as warm as possible, no draughts.
• Over-extrusion. By default, the printer is set to 200 steps per mm for the extruder. Really, this should be around 147 steps per mm. In Repetier-Host, Goto Config-> Firmware EEPROM Configuration -> Steps per mm E: 147.0
Lastly, as a beginner, I would recommend starting with PLA plastic rather than ABS. It is a much easier plastic to print with, makes almost no smell and will stick to an unheated Buildtak printbed much nicer than large ABS parts.