Gaps in the walls of my prints

I just finished building my prusa i3 and every time I try to print something I get gaps in the walls of the prints. To me it seems like the extruder isn't putting down enough filament on those spots. If you have any advice i=on what is wrong it would be very helpful.

Calibrate it

http://reprap.org/wiki/Triffid_Hunter's_Calibration_Guide

There might be following possibilites:

1) They Key of extruder might be broken

2) Filament Diameter may not be consistent.

Check the diameter of filament with guage and feed the same diameter in 3D Printing Slicing software.

Also if You are beginner, i suggest to start printing with PLA 1.75mm filament.

Also make sure to keep your filament enclosed in airtight container to prevent it from moisture.