For sharp images, you need the right combination of:
- Shutter speed
the first three are critical, it makes no difference what lens you have without the first 3.
-Focus: your focus point must be exactly where you want focus to be. Do not 'focus and recompose', meaning getting focus then moving the camera to compose the shot. Instead, compose then choose the appropriate focus point in your camera. You may need to test your camera/lens combination for accurate focus. Look here for instructions.
-Aperture: aperture is critical to good focus, because it impacts the depth of field and the circle of confusion. What this means is that as you open the aperture of your lens (say f3.5), the depth of what is in focus gets shorter. If you have a small aperture (f16) more depth is in focus at one time. An example is if you focus on a model's nose, with a wide aperture, often that model's eyes will be out of focus. Get to know and use a DOF chart.
-Shutter speed: This is less of an issue with a tripod, but shutter speed is critical for sharp focus because any camera shake can cause blurring. Generally, it is recommended to use a tripod for speeds less than 1/60, or 1/ your focal length (if shooting a 500mm lens, you need a speed of 1/500 or greater to eliminate shake from hand holding the camera). If your subject is moving, you need to increase the shutter speed to reduce blurring from subject motion. If you are shooting multi-second exposures, you can sometimes see blurring from the movement of the mirror, though that can also mean you have a bad tripod.
-Lens: if you have mastered all of the above, and still don't have sharp images, then your lens is to blame. The difference in image sharpness between a professional ("L") Canon lens and a kit lens can be shockingly amazing. However, note that this sharpness is usually only really apparent when zooming into the image. Sharp images look good at every perspective, but pixel peepers are really those that zoom in 100% during post editing, and declare whether a lens is truly 'sharp'. Get a new lens only when you have eliminated 1-3 above.