If I fell in a grocery store due to water on the floor and there where no wet floor signs any where, what do I need to do if I hit my knee pretty hard?

You need to file an incident or injury report with the store immediately, if you have not already done so.  Make sure the store is aware of the incident, when it happened, what happened and that you may have injured your knee in the fall.  If you have not already been to a doctor to have your knee evaluated and you feel that you need some medical attention, go to a doctor and have yourself and your knee examined.  If you have injuries and medical bills relating to the fall, you are going to want to establish a claim with the store's liability insurance carrier.  It is best in this situation for you to get the advice of a personal injury attorney.  I would strongly suggest that you contact one.  Most personal injury attorneys offer free consultations.  Good luck to you.