Hi,
If you're struggling on Chieftain difficulty, there must be some basics that you aren't yet aware of. Let me give you a quick list of basic tips that should help you out up to Noble at least!
1. Build more workers. I can almost guarantee you aren't building enough if you're struggling at this level. At least one per city is usual. When the game starts, research worker technologies (agriculture, pottery, wheel, bronze working) in order of importance. If you have corn and rice around you, get agriculture and build a farm ASAP. If you have flood plains, get pottery for cottages.
2. Don't be tempted to found a religion! Researching these technologies means you aren't improving your workers. You can always found a religion later, or capture a holy city for yourself.
3. Consider carefully before building wonders. If you'e creative, is it worth building Stonehenge? Likewise if you'e aa coastal empire, I'd strongly consider building The Great Lighthouse. Don't just build a wonder because they're available.
4. Pick your targets carefully. Don't strike at the most powerful empire straight away if you can't handle it. Pick on a weaker neighbour, take his developed cities and improve yourself that way.
5. Found new cities carefully. Cities should ideally have a food resource. Alternatively, they can be used to acquire an important resource, such as copper, iron or gold. Additionally, you can just plant a city somewhere to prevent a rival from expanding in that direction. Just make sure every city has a purpose!
6. Play to your civ's strengths. An imperialistic civilisation should be churning out settlers to get land ahead of your rivals. If you're aggressive, use your combat bonuses to crush your opponents.
7. Siege weapons are your friend. Destroy the enemy defences and then decimate their armies with them. Then mop up with your soldiers.
These should get you started. Give us more details about your game for more advice!