No decorative surface is more colorful or interesting than ceramic tile. Unfortunately, tile grout is not an easy-to-clean surface.
Although commercial grout cleaners are available at stores, it is easier and less expensive to use common household products to do the job.
For colored grout, vinegar works wonders--either straight or in strong solution with water.
Some people find that household bleach does the job.
For white grout, try hydrogen peroxide. Hydrogen peroxide is inexpensive, and it's safer to use than bleach.