You have to do a lot at once usually. I like CapStar for immediate relief (Nitenpyram) It is less toxic than many other flea meds and kills adult fleas within 30 minutes. You mix it into their food and it provides immediate relief. Sadly, it doesn't kill the eggs, but if you dose them every few days it will eventually stop the life cycle.
Take their bedding and wash it - or even throw it away if you have a bad infestation. Vacuum twice a day and after you treat for fleas go through and steam clean carpets as well. This is a non-toxic way to kill. If an infestation is too bad though you may have to resort to using a chemical flea killer on the furniture and carpets as well.
Once you get rid of any fleas your pet has, put them on a maintenance medication that keeps them from getting them in the first place.