Hi Alessio,
"Aim at" means to direct whatever it is we are shooting for at a specific target, whereas "aim to" can be about directing toward a general goal. Usually, "aim at" is used for physical targets; we seem to always use "aim to" when we're talking about non-physical targets.
Here are a few examples:
"I'm going to aim my arrow at target number two." In this example, the speaker has a choice of targets and is specifically choosing target number two to direct his arrow toward.
"He will aim to be the top student in his class." In this example, the person has a goal of being the best student in his class, but since the goal isn't a physical object, "aim to" is used.
"We cannot aim our missile at that ship! It's a friendly ship!" Once again, the target being referred to is a specific, physical one.
"Our team will aim to score at least sixty points per game this season." Once again, the goal is not physically visible.
There is also another "aim" expression that's sometimes (but not always) used for time-related goals: "aim for." For example:
"I will aim for 6:00 to come and see you, but traffic might delay me."
I hope my answer has helped. Please let me know if it didn't.