How do I beat the AI on Moderate ?

I am struggling against it. So far, i have fought it 3 times and I lost all of them. Here it goes...

1st battle

My civ : dont remember

I underestimated the AI and i got defeated

2nd battle

My civ : Slavs

I managed to repel the 1st attack but then fell to the next.

3rd battle

My civ : Teutons

It was on Arena. I repelled 3 massive attacks but lost due to lack of gold.

What should i do ??? I am just a casual player.