// SPDX-License-Identifier: GPL-3.0

pragma solidity  ^0.6.0;

// Solidity program to deposit eth and get contract balance
contract contractInfo {
   
    //  Defining a function to  deposit eth
     function deposit(uint _amount) payable public{
         require(msg.value == _amount);
     }
    
    // A smart contract can access its own balance as address(this).balance 
    // Given account's address, its current ether balance can be accessed as address.balance
    function getBalance() public view returns(uint){
        return address(this).balance;
    }
}

1. In ContractInfo.sol first we need have declared SPDX license that simply means that your smart contract is public and  can be used by anyone without your permission.
2. In the second line of code i.e. pragma solidity ^0.6.0; we have declared the compiler version.

3. Declaring a contract name contractInfo  using keyword contract.

4. Defining  function to deposit eth  to smart contract that takes only one parameter _amount of type uint  having visibility set to public. We are making this function payable since we want make a transaction that will cost us ether. Inside the body of function we are checking  if the required  condition is fulfilled or not since it is costing us ether we need take extra care so we are adding this condition msg.value == _amount using keyword require which is fall back function that will return our ether if the transaction is not completed. 
5. Defining another function to get the balance of smart contract . We are setting the visibility to public and also have restricted the state mutability by adding view keyword. This function is returning the balance of contract to do that we have added keyword this which refers to balance of own contract.