or you can "treat" yourself like i did with a very small portion of last years tax refund. i called serv-pro and found out they charge very little to send a couple of ladies out to get on their hands and knees and do that hard work. i was amazed that they charged so little (i have a long tile entry way and a large tile kitchen-it cost me less than $300 and was well worth it-the grout was 7 years old and despite my best clean up efforts had grape juice stains, punch stains....). my grout looked like the day it was put in. you can then follow it up with a sealant.
i started to do the "dirty work" myself with a clorox bleach pen-which worked great, but after working a few hours each day not much was done and i could barely walk.
p.s. don't call one of the advertised "grout cleaning" services they charge an arm and a leg as will any of the "merrie maid" type places. serv-pro has it down to a science and considers it an easy/inexpensive job.
if you're going to tackle it on your own try a bottle of "ultimate grout cleaner". this stuff is amazing-it got rust stains, coffee stains and worse out of my counter top grout with very little scrubbing.