An example of solidity. Copy and paste code into http://remix.ethereum.org/ and use a JavaScript VM runtime environment.

HelloWorld.sol

pragma solidity ^0.4.24;

contract HelloWorld {
    string public message;
    int256 public myNumber;
    address public owner;
    
    constructor(string initialMessage) public {
        message = initialMessage;
        myNumber = 42;
        owner = msg.sender;
    }
    
    /* 
    * Function modifiers are used to conveniently modify function behavior.
    * This modifier makes it so a function can only be called by the contract owner
    */
    modifier onlyowner() {
        require(msg.sender == owner, "You are not the owner of this contract");
        _;
    }
    
    /* 
    * Functions that modify the state of a contract cost gas.
    */
    function setMessage(string newMessage) public {
        message = newMessage;
    }
    
    /* 
    * View functions promise to only view state but not modify it.
    * notice how the modifier is also restricting access to this function.
    */
    function getHalfOfMyNumber() onlyowner public view returns(int256) {
        return myNumber / 2;
    }
    
    // Functions can be declared pure in which case they promise not to read from or modify the state.
    function getTheNumberSeven() public pure returns(int32) {
        return 7;
    }
    
    // Functions that are payable hold value in "msg.value". The contract then holds the balance
    function sendContractSomeMoney() public payable {
        require(msg.value > 0, "You must pay more than 0 wei");
    }
    
    // We can read the balance of a contract and return it in Wei
    function contractBalance() public view returns (uint256) {
        uint256 balance = address(this).balance;
        return balance;
    }
    
    // We can take money out of the contract too. 
    function recieveContractMoney() public payable {
        uint256 balance = address(this).balance;
        require(balance > 0, "Contract will not send you 0 wei");
        msg.sender.transfer(balance);
    }
    
    // This is how you lose money
    function badSendMoney() public payable {
        uint256 balance = address(this).balance;
        require(balance > 0, "Contract has no money");
        address badAddress = 0x123abc;
        badAddress.transfer(balance);
        
    }

}