I tripped over a pallet in a grocery store yesterday and today I caN barley walk and walking now has me in excessive pain.. .. What can I do

I will try to give you some basic advice, but it is not easy to cover everything in a short reply. For that reason, if you feel the incident was caused in part or whole by the store, I strongly recommend you at least consult with a qualified and experienced attorney in your area. When choosing a lawyer to consult with, be careful. Just because someone has a flashy TV add, does not mean they are good lawyers.  For the general advice: It is important that you document what happened. Take pictures, write down who you spoke with at the store and exactly what happened so you don’t forget. Be careful about giving any statements to a representative of the store, or their insurance company. There is nothing to be gained by giving them a statement, since most companies will act like they want to help you, but they will ultimatly deny responsibility ( they may offer you some small money to go away). Be careful. Next, you should get medical care/evaluation of your injuries.  It is critically important that you document all your injuries and symptoms. That does not mean to make up or exaggerate, but you need to be specific. For example if you have back pain that seems to be radiating down a leg, do not just write down "Back pain". You need to say I have back pain that is going down my leg.  Same for other parts of your body and you need to document symptoms that you are having, even if you are not experiencing it at the moment of your visit (some symptoms come on, based on activities).  If you do not want to involve a lawyer, at a minimum send them a fax (with fax receipt confirmation) and/or certified letter demanding they preserve any videos and still photography of you and the area of your fall, for a few hours bf and after the incident.  Again, these are just a few things that an experienced lawyer in your area will be able to better help you with when they understand the facts in greater detail.

Ask for incident report be made and obtain a personal injury lawyer.  The store may agree to pay for medical expenses if it thinks it was at fault but this is the exception.   take photos and do not delay

First you should seek medical attention asap. Second don't talk with the store or any representative that  contacts you. The store can only be responsible if they were negligent in causing your injuries. Hire counsel to investigate. Most of us work on a contingency fee basis.