If you want to rent chairs and umbrella when you arrive just approach the management of the concession and they will take care of your needs and give you what you need. By law they must post the prices of the various furniture or services you intend to rent, so make sure you look at it.
Sometimes when we were kids we would take some free ones left by departing customers and hope to get away with it. If you do after a while you might see a staff member coming to ask you to leave or pay the rental.
There is no obligation to pay for the rental, or to pay to access the sea. Bathing establishments that have been given the concession must guarantee free access to the sea and the strip of beach within 5 meters from the so called 'battigia' is not subject to the concession and must be free and unincumbered for the free enjoyment of everybody. That is public land. The 'battigia' is the water's edge where the wave s hit the beach. From that point to 5 meters inland, that strip of beach must be left free and must be made accessible. No umbrellas and chairs can be put there, but you can sit there on your beach towel.
Now, there will smart pants (not necessarily affiliated with the concession establishment) who will try to extort money from you even if you just try to access that free strip, or even if you sit down in that free strip. Unless you want to rent their services, just refuse and remind them of the law I just explained. If they insist tell them to call the Carabinieri. Knowledge is power. Once they know you know, they won't bother you.
Occasionally they try with me because they assume I'm an American tourist maybe because my black Puertorican wife can hardly pass for a local. I can't write here what I tell them because this is a family website, but the threat to call the carabinieri is enough for them to disappear from my face the rest of the day.
Also the Liguria Region has established very comprehensive guidelines on how much of a beach can be given in concession and how much must be left totally free. As I understand is 50/50 split, therefore you should be able to see a totally free beach next to the portion given in concession to the bath establishment. In any case even the portion given in concession excludes the 5 mt strip from the battigia (water's edge). Establishments try to cheat on the 5 meters, so be ready for that, it's Italy after all and the Monterosso beach is narrow.
The beach is not white sand like Florida. It's sand or maybe fine gravel type of beach, but not big river rocks and boulders like the other 5terre villages.