Parameters

address, 20 Bytes - hex encoded address of the storage.
1. e.g.) "0x295a70b2de5e3953354a6a8344e616ed314d7251"
QUANTITY - hex encoded integer of the position in the storage. 0 is for first position, 1 for second position.
1. e.g.) "0x0"

🚧
Calculating the correct position depends on the storage to retrieve.
Consider the following contract deployed at 0x295a70b2de5e3953354a6a8344e616ed314d7251 by address 0x391694e7e0b0cce554cb130d723a9d27458f9298.
contract Storage {
    uint pos0;
    mapping(address => uint) pos1;
    function Storage() {
        pos0 = 1234;
        pos1[msg.sender] = 5678;
    }
}
To retrieving the value of pos0 ("1234"), "0x0" is the right parameter.
However, retrieving an element of the map is harder. The position of an element in the map is calculated with:
keccack(LeftPad32(key, 0), LeftPad32(map position, 0))
This means to retrieve the storage on pos1["0x391694e7e0b0cce554cb130d723a9d27458f9298"] we need to calculate the position with:
keccak(
  decodeHex(
    "000000000000000000000000391694e7e0b0cce554cb130d723a9d27458f9298" +
      "0000000000000000000000000000000000000000000000000000000000000001"
  )
)
The geth console which comes with the web3 library can be used to make the calculation:
> var key = "000000000000000000000000391694e7e0b0cce554cb130d723a9d27458f9298" + "0000000000000000000000000000000000000000000000000000000000000001"
undefined
> web3.sha3(key, {"encoding": "hex"})
"0x6661e9d6d8b923d5bbaab1b96e1dd51ff6ea2a93520fdc9eb75d059238b8c5e9"
In this case. "0x6661e9d6d8b923d5bbaab1b96e1dd51ff6ea2a93520fdc9eb75d059238b8c5e9" is the parameter.

QUANTITY|TAG - integer block number, or the string "latest", "earliest" or "pending", see the default block parameter.
1. e.g.) "latest"

curl https://ethereum-mainnet-archive.allthatnode.com/8U3JLUhzIDg3GShvy9hkCCSYkLGc11kj \
--request POST \
--header "Content-Type: application/json" \
--data '{
    "jsonrpc":"2.0",
    "id":1,
    "method":"eth_getStorageAt",
    "params":[
        "0x295a70b2de5e3953354a6a8344e616ed314d7251",
        "0x0",
        "latest"
    ]
}'

Returns

DATA - the value at this storage position.

{
    "jsonrpc": "2.0",
    "id": 1,
    "result": {
        "0x00000000000000000000000000000000000000000000000000000000000004d2" //value of the storage psition.
    }
}

Try Yourself