I don't even know where to begin here.

In school i learned the the League of Nations biggest flaw was that it had no army

The United Nations/League of Nations is/was an intergovernmetal organization intended to promote international cooperation, it is not a military organization, nor was it ever intended to be one. Saying that their lack of a standing army is a weakness is simply unfounded.

but the UN has one

This is plainly untrue. The UN has no standing army what so ever. When peacekeeping missions are needed, soldiers are provided by the various member states on a voluntary basis. Last I checked the current force was around 90 000 uniformed personel but these men and women are all still part of their respective national armies.

yet after WW2 it was only USA participated in major wars

This is patently untrue by every single definition.

I feel like they are a bit useless.

While there is criticism, partially valid criticism, of the effectiveness of the UN peacekeeping missions they have historically been far more successful than the "nationbuilding" efforts of the US (same thing, different name). At present, 7/8 nations where the UN has concluded peacekeeping missions are at peace compared to 4/8 for the US "Nationbuilding" missions concluded.

("The UN's Role in Nation Building: From the Congo to Iraq". 2005. A Rand Corporation)
("The 2005 Human Security Report" 2005)

How is the UN helpful

They remain the principal agency for the peaceful resolution of international conflict today.

and are there any significant actions they took?

The United Nations Peacekeepers have concluded 55 missions since its foundation, some peaceful, some quite violent. There are 18 active deployments at present. It is however important to note that the goal of the Peacekeepers is to stabilize a situation, not escalate it into full scale war. For this reason, the mandate of the UN Peacekeepers rarely include offensive operations or direct support of one side of a conflict.