How to get dirt and oil stains out of white pant?

I have white cotton pants that has dirt stains from a lot of use.. In certain light the pants look totally white, but in other lighting you can see the filth. Also, I spilled olive oil on them from pizza, and I don't know if I can get that out or not.. Can a dry cleaner get the stains out maybe, or, is it even possible to get such stains out myself?