One of the great things about Italy is that its long thin shape means that when you get fed up with sightseeing you are but a relatively short distance from a beach. But when you get there you might be rather confused, especially if you come from a country where beach access is free to all.
In theory that is the case in Italy but as with a lot of things in this country the practice may be somewhat different to the law. Many stretches of beach, particularly those close to urban areas, are let out to private concessions. In the season they cover almost all the beach with rows and rows of sunbeds (lettini) and umbrellas (ombrelloni). In theory you should be able to pass through these establishments to get to the sea, and should be able to walk along the sea in front of them, but you may be prevented from doing so. In places like Capri or the Amalfi coast you might be charged up to 10 euros even just to step on the beach or to swim in some caves! More affordable are the beaches in Calabria, most are free, you will only need to pay for the eventual equipment you want to rent.
While renting lettini for the day is not particularly expensive the establishments can fill up very quickly. There are some free beaches everywhere and these may be your best bet. They are easily identifiable by the absence of regimented rows of lettini. They can get very crowded: on a Saturday or Sunday in the summer you won't find an empty stretch of beach anywhere. Close to urban areas you will never be far from a fish restaurant on the beach or, at the very least, a bar. On the beach, topless women are more or less accepted everywhere but nudity is limited to certain beaches. These are unlikely to be announced as such, so you will have to be guided by what others are doing.