You can find 100 gold coins on the Bjornholm map. There are spread around the whole map and finding them all may take you some time. Some coins are easy to spot and some are hidden, for example behind building or between trees. It is good to have a car, when looking for the coins (however, buying a Piqup is quite an expenditure, as it costs 59 000$). Remember also that as a farmer, you are in great shape, so you can sprint all the time ([SHIFT]). After collecting all the gold coins, you can collect the reward.
A coin will be picked up automatically when you get close to it. You will be notified with an icon in the top left corner of the screen. After collecting 10 of them, you will notice yellow spots on the map that mark the location of the coins. You obtain achievements for collecting 25 coins, 50 coins and 100 coins.