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Bets.sol

pragma solidity 0.4.25;

contract Gold {
    function remoteApprove(address /*_to*/, uint256 /*_amount*/) external pure {}
    function transferFrom(address /*_from*/, address /*_to*/, uint256 /*_amount*/) external pure {}
    function balanceOf(address /*_user*/) external pure returns (uint256) {}
}

contract MainBets {
    // the number of block after which the battle may be started
    mapping (uint256 => uint256) public battleBlockNumber;
    // has the battle occurred?
    mapping (uint256 => bool) public battleOccurred;
    
    function getBetCurrency() external pure returns (bool /*isGold*/) {} // false - ether, true - gold
    function isCreatorWinner() external pure returns (bool) {} // false - opponent is winner, true - creator is winner
}

contract ExternalBets {
    Gold goldTokens;
    
    function _checkTheUserHasEnoughGold(address _user, uint256 _value) internal view {
        require(goldTokens.balanceOf(_user) >= _value, "not enough gold");
    }
    
    // Example of bet in gold
    function _getGold(
        address _user,
        uint256 _bet
    ) internal {
        _checkTheUserHasEnoughGold(_user, _bet);
        goldTokens.remoteApprove(this, _bet);
        goldTokens.transferFrom(_user, this, _bet);
    }
}

contract Proxy {
    ExternalBets externalBets;
}



// The task: fill ExternalBets contract with the next functional:
// 1. make a bet by battleId on one of the dragons. Allowed if:
//    a. the opponent is selected (battleBlockNumber > 0)
//    b. we don't know the result (battleBlockNumber > block.number),
//       because we use hash of future block for random number generation
//    c. the battle has not occurred (battleOccurred == false)
// 2. update your bet. Allowed if:
//    a. we don't know the result (battleBlockNumber > block.number)
//    b. the battle has not occurred (battleOccurred == false)
// 3. return your bet. Allowed if:
//    a. we don't know the result (battleBlockNumber > block.number)
//    b. the battle has not occurred (battleOccurred == false)
// 4. get reward. Allowed if:
//    a. the battle has occurred (battleOccurred == true)
//    b. your bet won (isCreatorWinner() == true, if you bet on creator,
//                     or isCreatorWinner() == false, if you bet on opponent)
//
// Data (should be public):
// 1. array of user bets (battleId, bet size)
// 2. two arrays of bets for specific battle (user address, bet size)
// 3. two sum of bets on each participant
// 4. maybe something else, that would be needed/helpful
//
// All external bets (ether, gold) and calculations should be stored in ExternalBets contract
//
// User will call methods of Proxy contract and Proxy will call methods of ExternalBets
// So you should transfer ether across Proxy contract to ExternalBets
//
//
// The reward is calculated as (sum_bet_on_2nd_dragon * your_bet_on_1st_dragon) / sum_bet_on_1st_dragon,
// if I think correctly

Any resource material you can recommend for a beginner?

@charmingseo, sorry, but I can't recommend you anything because I was a beginner a year ago and many things changed during this time. Just google it :)