You meed to provide much more information. Silver half dollars were made for almost two centuries at about a half-dozen mints so at a bare minimum you need to identify its date, condition, and mint mark.
If it's dated 1965 to 1969 it's made of 40% silver and is worth about $2.75 to $3.00 as of 02/2010.
If it's older than that it's made of 90% silver and its value is very dependent on its date and mint mark. If your coin is dated 1964 or earlier please post a new and more specific question. This is especially important for the Ben Franklin coins 1948-1963 and the earlier Liberty Walking halves.
If the coin is a US Kennedy half dollar dated 1974 it has no silver in it and is face value.
Value is for the silver about $2.50
No US silver dollars were made after 1935, look at the back of the coin for 'Half Dollar'.
With a date of 1979 the coin has no silver in it. 1969 was the last 40% silver circulating US half dollar. The coin is worth 50 cents.
The coin has NO silver and is face value
If the coin is a US Kennedy half dollar dated 1974 it has no silver in it and is face value.
Value is for the silver about $2.50
No US silver dollars were made after 1935, look at the back of the coin for 'Half Dollar'.
A US half dollar with a date of 1979 is a Kennedy half dollar not a liberty half dollar. The coin has no silver and is only face value.
There was no silver dollar in the US made in 1957. It might be a half dollar. These are usually worth about $10-15.
The coin is a Franklin half dollar (1948-1963) and is common. If the coin shows any wear, value is about $10.00 just for the silver.
With a date of 1979 the coin has no silver in it. 1969 was the last 40% silver circulating US half dollar. The coin is worth 50 cents.
The coin has NO silver and is face value
There is no silver in a 1972 US half dollar.
A US Kennedy half dollar dated 1969 is only 40% silver with a value of about $2.00.
Look at the back of the coin for "HALF DOLLAR" no US dollar coins were struck after 1935.
Please check the coin's date and denomination US silver dollar production was suspended between 1905 and 1920 inclusive, so there are no 1917 dollar coins. If your coin says HALF DOLLAR on the back, it's a half dollar, not a silver dollar.