1. Is it the sole plate that's dirty? Like there's residue on it? My mom taught me to take a large sheet of wax paper, fold in half a few times so it's 4 or 8 layers thick, but larger than the face of your iron. Briefly iron the wax paper and then iron on a clean towel. You'll need to repeat this several times, turning the wax paper to the flip side as necessary.
2. Is it the steam valves that are dirty? If so, check your manual. Mine has a "cleaning mode" that I can turn the steam dial to. I fill it up with water, heat for 2 minutes, then unplug and hold it over a sink while the water drains. I do this every so often when I get steam spots ironing.