Roasted coffee goes stale because of oxygen, light, heat and moisture. Keep it away from all four and it will taste good for weeks.
Buy whole beans and grind just before brewing. Ground coffee loses aroma within hours. Keep the bag closed with its zip, or move the beans to an airtight canister. Do not store coffee in the fridge, where it picks up moisture and smells.
If you buy a kilo, freeze some of it in small airtight portions and take out one portion at a time. Let it reach room temperature before opening so no water condenses on the beans.