I have just purchased an expensive espresso coffee machine (American) but the taste isn't nice and strong as I expected. Does it depend on the coffee beans? What characteristics should I look for (in the coffee or in the preparation)?
You do not tell us too much about your equipment, so I guess that you are in the same situation me.
I was in the same situation when I bought my machine. I purchased a Rancillio Silvia, without a grinder. I blamed it on the coffee at first, but it turned out that all the pre ground espresso coffee I was buying in shops was simply not made for such a machine. I was so lucky that I got to grind the my beans at a local coffee shop, using the same find grind as they did in their machines. Only then I got the crema and punch that I wanted from my espresso.
Symptoms to look for:
It didn't take me a long time to understand that I needed a own grinder to get exactly the espresso I wanted.
If your grind is just a little too coarse it is possible to counter this by increasing the tamper pressure. But from what you explain you need more than this.
I don't know what kind of coffee you use, but I think that any coffee powder specifically made for espresso should be fine, at least to start with. But, bear in mind that the coffee quality is not the only factor that makes the good espresso, it counts a lot, but there are many other aspects to take into account.