Homemade grout cleaner
Mix the baking soda and bleach together in the bowl until it forms sort of a thick paste. Then apply the paste to the dirty grout lines and let it sit for 5-10 minutes.
The bleach will do most of the work just sitting there, but after 5-10 minutes, scrub it with a toothbrush to really work the cleaner deeper into the grout.
Wait another 5-10 minutes and then rinse the cleaner off the grout. If you have a handheld shower head then use that, otherwise just use a damp rag and rinse it out often as you wipe the cleaner away.